v1 iPhone, yes, you can do that; you would need to get the voice plan from AT&T and a data plan for $20 a month.
3G iPhone, can only be activated with AT&T at present, so the Apple/AT&T people will have a new SIM card already installed and activation via iTunes by Apple/AT&T automatically bricks the SIM card that currently works with your assigned number; voice plan required and the data plan is $30 per month. Now, just go home and add your contacts, music, video, TV shows, podcasts, etc... You should be good to go. 😎