The iPhone 3G will be only available in stores in the United States. The activation of the new iPhone is not done over iTunes anymore. You have to go to a physical store to activate the new Apple phone.

UK gadget magazine T3 reports now the O2 and Carphone Warehouse will still sell the new iPhone 3G online.
The article does not specifically detail how the new iPhone 3G will be activated in the UK, but if they sell it online they cannot expect people to have to walk to an O2 store after they received the iPhone 3G in the mail.
Via T3. More about the iPhone 3G.

We get lots of search traffic for "unlock iPhone 3G". This shows that there is still a lot of interest in running the new iPhone 3G on any network and not just on exclusive carriers like AT&T, O2, or T-Mobile.