Not going to happen. There's a rumor that the iPhone 3GS will not run iOS 5. I think that's false.
If anything, it will not be able to take advantage of all the new features in the next iOS upgrade but it will still be able to run more than a few cool features that are coming our way
If I had to speculate right now, I would have to say any Siri integration may not work for the 3GS because of the higher level of processing power that is required. Of course, it really depends on Apple how they implement it. After all, Siri works even now on just fine.