This guide no longer applies if you purchase an unlocked phone from the USA. However, just a warning...if you upgrade your firmware, the lock will be applied again and the phone will be useless.

Hi. This is for all you people who do not reside within the United States of America. I purchased an iPhone and had it delivered to my home in Hertfordshire, England from New York, USA.
THIS PHONE DOES NOT ACCEPT ANY UK SIM CARDS...AND THIS MEANS THAT O2, VODAFONE, ORANGE, T-MOBILE, 3, VIRGIN...
...whoever may be your network provider, you cannot use this phone here.
I was lucky enough that the seller was reasonable, and failed to mention that in the auction and so i recieved a refund.
AGAIN, this phone will not work in the UK...or anywhere outside USA for that matter that isn't supplied by AT&T.
PLEASE PLEASE PLEASE do not buy this phone as you cannot get it unlocked as of yet (Friday 6th July 2007).
The phone is effectively double locked...
There is a SIM Lock - which means only AT&T customers can use it
There is also a program lock which means you have to activate the phone using a itunes style program...which doesn't work from outside the UK.
Hope this helps someone