I had to jailbreak mine even using AT&T. Nothing compares to running real apps on the iPhone, not even the web apps.
I've got a UK (OOB 1.1.2) unlocked and upgraded to 1.1.3 on a 'non-official' O2 Simplicity Tariff.
LOL my son cracked the screen of his iphone yesterday, so bought him the new 16G and took his cracked on a member sent me the unlock app...and unlocked it....now doing another...IF ATT was no so crappy where i live, i would be good but now loving t mobile......