iPhones from different carriers are locked to that carrier, so the answer is no, not out of the box, BUT you could SIM-unlock/get it SIM-unlocked. The lock has been cracked (looks like up to fw v.1.1.1, uk 1.1.2) and people are selling them on ebay, and there is also a shop on oxford street in London who claim to do it for a �sum and it works and doesnt need to be activated with AT&T, check their FAQ on (google for iphoneunlockuk), you can call them and explain/ask advice before you commit cash to this idea.
Good luck and perhaps let me know how you get on? I would love an iPhone two, I could abolish my Nano and have a browser on my phone, w000t!