Google voice assigns you a phone number (currently only those that were Grand Central customers have one - GC was the company Google bought to make Google Voice). You can then manage where that number can reach you, at home, office, cell, etc. The phone carrier does not really matter as GV simply dials the phone number.
Google voice will either pass the ANI or send your own GV number as the caller ID for calls coming through GV, which is pretty cool.
Another added benefit is that GV can also accept SMS which is then forwarded to your cell phone or you can reply to those messages online.
If you get a voice mail on GV, you can chose to either be notified by email or SMS to your phone.
Hope that helps.

Actually, I think I figured this out...

You would simply add your AT&T cell phone number to Google Voice, and your cell phone would ring if someone called your google voice number. I'm not quite sure how it would notify your cell phone that you have a new voice mail though. I don't imagine it would be the same as getting a voice mail through AT&T.

Google Voice is currently limited to only Grand Central customers (whoever they are).

I'm sure they [Google] will be looking for expansion soon.

I heard about Google Voice today and I'm wondering how it is going to work with AT&T. Anyone know?