Some clarifications here:

1. While RIM can in theory access the data going through their servers on the way to the wider internet, I've never heard of anybody hijacking a blackberry through the mobile network, let alone using such a hijacked blackberry to get into someone's wifi network. I suppose it MIGHT be possible in theory, but there is almost zero risk of it happening to you. There are a billion other things you should be more worried about, which is why people are calling you paranoid. If you're concerned about network security, don't worry about a blackberry being on your network. Just change the password every so often and don't give it to people who don't have a need to know it.

2. Your wife's e-mail account getting hacked almost certainly has nothing to do with your network security. E-mail accounts usually get hacked either through brute-force password guessing, or through your wife doing something stupid: falling for a phishing scheme, accidentally downloading malware onto her computer, using another computer to check her e-mail that is infected with malware, etc. (note "doing something stupid" doesn't necessarily mean she is stupid; some of the most savvy internet users out there fall for these tricks now and then).

Please do not put words in my mouth. I did not say it had anything to do with network security. I simply stated it happened. That's all. Something was obviously overlooked.
Are you saying there is absolutely no way for a an irresponsible/lackadaisical user of blackberry to make a network or other computers on the network vulnerable?
I have said it before and I will say it again, the question isn't just about the Blackberry, it's about blackberry in the hands of a user who is not responsible. Or are you saying that Blackberry user can/wlll never do anything stupid?

No. Just no known way has ever been discovered. The functions each application has access to are secured within an envelope that allows certain functions to be permitted by the user on first run. Things outside the scope of actions allowed in the BB OS's security management envelope wouldn't be possible at all until someone manages to root the BB OS, which unlike the iOS which gets rooted in a new way every time a new one comes out, has never yet been successful last I checked.

Based on all the inputs here, I gather that given the level of the personality shown by the person I am talking about, he himself is a bigger risk than the blackberry itself.
I understand that. But the reference to paranoia in this situation is unwarranted, in the light of the fact I am talking about a specific situation where one user has no respect for privacy and takes snooping into another person;s computer lightly.
Thank you all for your input.

My first helpful post got deleted because someone incorrectly assumed it was a personal attack. It wasn't. You seem very much overly concerned about security here...to a degree that to me borders closely on obsession. I've helped several people setup networks and deal with securing their house from that perspective.

First:

That was most likely a virus. There aren't teams of millions of hackers taking the time to break into one machine. They create malware and get that to do it for them. You wife probably clicked on some things she shouldn't have or installed software she though was ok. The chances that someone took the time to that personally are extremely small. You're not that big of a target.

Second:

You either don't mind the relative's son being on your network or you do. You have that choice to make. You seem to want to micromanage that decision and it's the wrong approach. It's either ok in your world or, if it causes you that much anxiety, it's not.

If it's not, make him spend the money to get his own air card or his own internet setup in the house.

If it is, then give him access. Are you a secret agent or something? If you're a normal person and not running NASA then you have little to fear by letting him on your network.

Third:

The likelihood of the Blackberry causing a problem is very small. Him streaming Pandora over your router isn't going to cause someone to hack into some database to change the title of your house to someone else. All of that stuff you see on TV where people hack into various agencies in 2 minutes whenever they want is fiction. It's called filling plot holes. It's not real.

Fourth:

You're letting the guy use a PC on your network. That you'd be worried about the Blackberry makes no sense. The chances of a PC getting infected with something? Very high. The chances of a phone getting infected? Low. He wants his Blackberry on your network because it is faster. A PC is what is likely to be infected with malware, trojans, etc.

Fifth:

You seem extremely concerned about security of your wireless network. Get rid of it. Wire your house. Someone then would literally have to gain physical access in order to use it. Again, unless you're worth $10m it shouldn't matter. It's not like you should have anything that secret on your computer hard drive. Also, in the case where you are a high net worth individual, stop reading this post and go hire a local firm to set all this up.

Sixth:

If you have this much anxiety about any other things in your life and it is common for you to have more concern about many things than a normal individual generally does, you may want to chat with it the next time you have a doctor's appointment. People who micromanage things in their life, are excessively worried about others and have fears that don't bother other people often have issues with anxiety. Should that be the case, then it would not be a bad idea to deal with it.

Seventh:

Have you wife switch to Gmail. It's more secure and the folks at google keep it both spam and hack free. It also gets rid of the need for an address book just for email addresses. That people use the built in Outlook target for email is just a bad idea.

Eight:

A wireless network is ALWAYS insecure. It's just sitting there. No matter what you do, someone can get on that network. You can hide the SSID, restrict the Mac addresses, turn down the radio strength, use WPA2 and the best network settings. Someone can still get on it. The most likely case that this happens is that one of your neighbors doesn't want to pay for internet and uses yours.

Ninth:

You seem very concerned about people hacking your network. You might want to take backups of everything and keep things like Quicken, tax returns, etc, off of your network. You might also want to find some software that will make sure no personal information is left on any machines.

I think that you're missing there is that it is theoretically possible, but the chances of that happening are enormously small. Know what is likely. Someone hacking windows. The chances of that happening are degrees higher than someone hacking a blackberry. By the way, they wouldn't hack a blackberry to get in to your PC. Regardless, that Blackberry is the most secure thing in your house. That someone who would think it bad to put a Blackberry on a network and ok to put a PC on the network is rather silly. The latter is the problem.

Your wireless network is one massive gaping hole in your 'security'. Your wife's PC is another one as is every PC in your house. The blackberry is the least of your concern.

Aren't there thousands and thousands of pieces of malware released for PC's each year? Didn't your wife's computer get 'taken over'? Did you take that offline and not let it in the network. People here are clearly telling you that the Blackberry is not something you should be worried about? Why even post if you won't accept the answer?

not when there is no routing between the nets. So it all depends on what you are using as the firewall. For traffic to pass between networks, something has to perform the routing function. A firewall is just a router that has rules to manage traffic is able to be routed - o.k. that is a very simplistic description.

So there is no way to route between the various SSIDs except for what I want to allow. And my "guest" network cannot route to any of the other SSID nets. My guest network is just as much "behind" a firewall as the other nets.

Not true. I have yet to hear of people cracking into WPA2 AES protected wifi networks using strong passwords.

At the current time, WPA2 AES wifi is probably secure enough for pretty much all residential users.