A little late, bit maybe this will help:
United Mobile (formerly Riiiing) is a Swiss company with a Lichtenstein (FL hereafter) based SIM (your phone number will be +423-663-xxx). They have free incoming calls in the EU and most other western European countries plus Oz and some others. Of course, whoever calls you pays and there sometimes are surcharges to FL.
CallBackWorld.com (CBW) is call back service. You are assigned one or more US trigger numbers. When you (or a wrong number
) call the trigger number and hang up, their system will call you back at a predetermined number. (Since the call isn't completed, you generally are not charged at this point).
When you answer your call back, you receive a pleasant voice that says "Dial your number" and after you enter it you are connected to that number.
If you are in a country that has free incoming calls (or at a land-line) you are not charged for the call back and just pay whatever the CBW rates are. They have a fairly low rate to FL ($.14/min) and they bill in 6 second increments. So with a little bit of work you can call fairly cheaply back to the US from your mobile.
They have another service that I find very useful -- a toll free number. There are two versions of this: The "free" one (you pay connect time but no other fees) is a shared toll free number. You are given a PIN which you give to people who will call you. They call the toll free number, enter your PIN and they are connected to whatever number you have setup -- in this case probably your United Mobile number. They also have a "business only" (though what is a business is loosely defined) that has a monthly fee of $1.50 and you get a dedicated toll free number that connects directly. In both cases the rate to FL is $.14/min
The other good thing is all these preset numbers (call back number, forwarding number for toll free) can be set from their website so you can change them as you move around. I spend a lot of time in Germany so I have a German SIM but when I am elsewhere I used the United Mobile. With this, people can reach me fairly seamlessly no matter where I am.
Now the caveats: CBW has been having problems of late with their network provider (I think it DialThru) and there have been periodic outages. Its not something I would depend on if you absolutely, positively have to have a call connected. I always have a backup plan. There can also be some annoying delays when talking. But all in all it has been a cost effective way for me to keep in touch with people and vice versa.
I echo what Dubai Stu says about PrePaidGSM.net -- it is a great resource for not only the variety of "international" SIMs but also it has details on many national offerings. It is the "flyertalk" of the prepaid GSM world
As for your Blackberry, I can't help you there.