If Apple [AAPL] impressed you enough to grab an iPhone 4 in the next couple of weeks then you might be able to save a bit of money by selling your old phone back to O2.
We don't yet know the iPhone 4 UK price, but we are hearing rumours that it will fall in line with what the iPhone 3GS prices were recently ie, if you pay more per month and commit for longer you'll get it for free and then the lower you commit, it will cost more out right.
On to the O2 deal, we noticed recently that they have a recycling scheme going on and the prices they are willing to pay back are not bad at all. Looking at the prices we see that the iPhone 3GS 32GB fetches up to £253 while the 16GB 3GS will pay you £240.
If you go to the recycling page and enter the details of your device you might be surprised with what you get. The process is very easy in that you select the manufacturer and model, agree to the price, fill in your details and select how you want paying and then post the phone to them.
What we do recommend you do is act quickly on this as prices could drop with the iPhone 4 announcement.