UK carrier O2 has announced their upgrade policy for the iPhone 4, including offers for existing subscribers who want to jump onto Apple's new smartphone ahead of their contract ending. Between June 24th and July 24th, existing subscribers still within their contract can upgrade to the iPhone 4 by paying £20 ($29) per month remaining in a one-off charge.
Alternatively you can transfer to pay-as-you-go and buy an iPhone 4 that way, though you'll obviously need to continue paying off your monthly agreement. Finally, there are early upgrade bonuses depending on where you are in O2's "Priority List" (which depends on monthly spend and loyalty); that can grant you up to a six month early upgrade.
There's an O2 early upgrade calculator here which works out how much you'll be paying depending on how close to your contract renewal date you are. Of course, with Vodafone, Orange and T-Mobile all planning to offer the iPhone 4 in the UK, there's likely to be plenty of competition for your business.
Apple iPhone 4 hands-on: