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).