If I purchase an unlocked iPhone 3GS, can I use with regular ATT 3g sim card?

[answerneeder]

If I buy an unlocked iPhone 3GS, will I be able to use it
I have a regular phone with ATT, will I be able to put that SIM card into the iPhone and use it? my regular phone is 3g, so my sim card is 3g
On my current plan(and SIM card)
I have minutes and unlim. texting.
If I ONLY talk and text using my iPhone, will I be charged my regular bill? Or will something still be added.

This means use NO internet, nothing other than talking and texting

[evilattorney]

ATT requires the data plan on all iPhones regardless of where bought, whether locked or unlocked, jailbroken or not. The data plan will be automatically added to your bill as soon as the network detects that you're using an iPhone.