As has happened already this year, the iPhone 4 is breaking websites. Three have not managed to escape this.
An SMS & E-mail was sent out recently to several/all? of the “register interest” folk which has in turn caused a lot of traffic to the main 3 website. 3 have done a very good job of separating parts of their site so a lot still works without problem.
O2 & Orange have dedicated websites, on different domains just to handle the iPhone, this should mean they have kept downtime to a minimum. O2 have also announced no online ordering until at least the end of July.