will buy iphone from france, can i unlock it/use it in the us?

You can look here or ask the carrier in France if they unlock iphones:

http://support.apple.com/kb/HT1937

Discussion of hacking iphones is not permitted here.

They do sell unlocked iPhones at full price - required by French law I believe.

The iPhone is sold in France as carrier locked with a designated carrier, and as officially unlocked iPhone sold at a significantly higher price than the carrier locked version.

If unlocked, the iPhone should work with an AT&T SIM card, or with any carrier's SIM card.

You should know the iPhone's warranty and support is honored by Apple in the country where the iPhone was sold officially by Apple only.

Hacking an iPhone for any reason is against Apple's license agreement for the iPhone, is not supported by Apple directly or indirectly via these discussions, and voids the warranty. And when installing a firmware update on an iPhone that has been hacked to be unlocked re-locks the iPhone to the designated carrier the iPhone was sold as carrier locked with when new.

If you purchase an officially unlocked phone from France, yes, it will work with an AT&T SIM with an AT&T plan (no roaming).

However the warranty will only be valid in France - i.e. if the phone breaks and needs warranty service it will have to be returned to Apple's repair center in France or a French Apple Store.