Apple is reportedly set to offer the iPhone 3G to Orange and T-Mobile when the 2-year exclusive period with O2 expires. O2 will retain the exclusive rights to the new iPhone 3GS though.
Orange and T-Mobile already offer the iPhone in other European countries and will be able to offer the 3G in the UK as early as September. It's no surprise that Apple would offer the iPhone to other providers and allow O2 to retain the exclusive on the new device.
I suspect that in the next year or so when the next new iPhone hits the market we will see the same thing happen to the 3GS. I bet the US gets the same treatment with the iPhone 3G as well.