Will my SIM card from AT&t work on a new iphone 3gs?

I currently have an iPhone 3G and my provider is AT&T. I'm currently looking to purchase an iPhone 3GS online, and I was wondering if I could activate the 3GS with my current SIM card now? The iPhone 3GS I'm looking at is also AT&T, and I have to check and make sure I can use my current SIM card before I purchase the 3GS.

It should work fine.

Yes, a SIM is a SIM is a SIM.