Well, O2 likely have the monopoly on iPhone for about 5 years (I think that's what it is with AT&T), or roughly anyway.
However, you can unlock the iPhone and use it with a 3 sim card. It's a bit hit and miss though, and you certainly void your warranty etc. You'll also lose other features like visual voicemail (O2's network was specially adapted to get this feature).
thanks for the advice, maybe will have to get an O2 sim then, I don't want an O2 contract at �35 per month when all I am paying on 3 is �15.
Also I know that the iphone doesn't support 3G !!!!!!!!