Having got all excited about the new iPhone last night it seems I may have jumped the gun a little. Unlike in previous years, there is no early upgrade path for existing iPhone customers. According to the O2 Website:
If you're already with us on Pay Monthly, you can upgrade to the new iPhone 3G when it's time to upgrade. It's usually the last month of your contract, but you can check at My O2.
So for me, that means no upgrade until the end of the year. Oh well, I shall have to make do with the new OS in the meantime.