have you tried it before? because supposivly blackberry internet plans use a different type of internet?
Yes I have. If you use BIS, and put your SIM in the iPhone, you will gain all internet access, just not the ability to use Visual Voicemail. If you use the iPhone data plan and put your SIM in the BB, you will not get internet.
You can just call AT&T, ask them to keep you on BIS if you want, and tell them that you will be swapping SIMs between the two phones. They may make note of it in the account and allow you to do this.