"Once the iPhone becomes available on other UK networks, we will allow O2 customers to unlock their iPhones, although of course they will still need to honour any outstanding contract period they have. At the end of their contract period, they are entirely free to move to another operator...."
Of course, this hinges on other carries -- namely Vodafone and Orange -- ending O2's exclusivity. Both Vodafone and Orange are scheduled to start selling the iPhone next year. O2 first announced UK pricing in June of this year.
That's good news for users in Britain and makes me wish the same were true in the US. While I love my iPhone, AT&T often ruins the experience with dropped calls and "fewer bars in more places." The thing is flawless on Wi-Fi; hit-or-miss via 3G. A part of that is likely due to my location (my neighborhood makes that town in Deliverance resemble Gotham), but I'm certainly not the only one complaining about coverage.