O2 already said on Thursday that stock of the iPhone 3G will be limited and that customers have to wait until August to get an iPhone 3G. Now after the iPhone 3G launch day is over the O2 website states that there are no iPhone 3G left in stock currently.

O2 says that every customer interested in an iPhone 3G will get one by the end of this summer. The initial supply is limited and will be for some weeks.
O2 is working closely with Apple to get additional iPhone 3Gs. These will be coming in on a weekly basis. Stock updates will be published on the O2 page linked below. I also recommend visiting an O2 store to apply some charm to a rep to get try to reserve a iPhone 3G - good luck.
Via the O2 site.