don't see it happening
I'd like it to. My 24-month contract from when i got the iPhone4 will expire in June, so I'd be keen on upgrading to the iPhone 5 at an O2 assisted discount (rather than the full price I paid for the iPhone4)
But I don't see it myself.
They've no need to rush it out. They'll hold out until it's about a year since the 4S launch and then announce the 5. By that time the anticipation and the hype will be unbearable and we'll all be dying to queue around the block outside the Apple stores again