If I were to bet on this (other than on the Euros!)
I'd reckon around oct time as this is when 02's current 18mth subscriptions end for iphone 1.
As already hinted in other posts, the reasoning being that;
When voda didn't win the first contract stated that 02 only had it for original iphone,
3gphone requirements states wireless carrier required rather than 02,
on the 3gphone pic all reference to 02 has been removed it only shows 3g on reception bar,
Why would 02 go PAYG,
It also states to check with wireless carrier as not all services may be available (ala visual voicemail??)
All in all, me thinks unlocked phones are a coming!!