The SDK actually leaked out to the developer community before the official WWDC announcement, giving some users a surprise peak at what was to come.
And the good news is that the 3GS will still be keeping up with the iPhoneses as Apple confirmed that the new software will be available to download for the legacy device.
Check out all the new features of iOS 5 in this handy video from our colleagues over at T3:
Getting in on the good stuff
This means features like the split screen keyboards for iPads, over the air software updates and tabbed Safari browsing are all coming to the legacy models for free.
However, you'll have to be ready no matter what the iOS device you're running, as Apple is only offering a 'Fall' iOS 5 release date for the new software.