It seems that Pocket-lint's anonymous tip was bang on the money, O2's upgrade microsite has confirmed that, for a limited time at least, it will cost existing customers (not just iPhone users) £20 per month remaining on their contract to buy themselves out. They can then take out a new contract with the provider for the new iPhone 4.
An O2 spokeperson told Pocket-lint: "Yes, you were right. We want to make it as easy as possible for people to be able to upgrade to iPhone 4."
It has also been suggested, by a reader no less, that you can also use O2 Recycle to sell your existing handset for anywhere up to £253 (for the 32GB iPhone 3GS). Essentially, that means, should you have 6 months left on your contract, and the iPhone 4 costs £250 new (as expected), you would only need to pay around £117- on top of a basic price plan - for a full upgrade.
Now that's not too shabby, surely?