i would get it on the bay quick as the prices are tumbling fast! 3gs is old news now mate
To be fair, the 3GS is still a decent handset. It will support iOS 4.
Unless O2 come out with some fantastic upgrade packages, I'll be quite happy to stick with my 3GS until next June. Lets face it, O2 have supposedly been doing very well out of the iPhone 3G/3GS and as such has a large share of the iPhone market. Its in their best interest to try and retain this market share with the iPhone 4.
I can see 3G handsets falling through the floor due to them being discontinued and not supporting many of the iOS 4 features. I notice they are listed as out of stock on the Apple store.