I have T-mobile now, and later this year I would like to switch to AT&T to get the new iPhone 3G. But I'm using an unlocked GSM phone right now.

I would still like to use my current unlocked phone when my iPhone is broken/unusable/whenever (after I switch servers).

So can I use the new sim card (that i will get from AT&T for the iPhone 3G) in my current unlocked cell phone?