Ngmoco say they have no plans to start releasing iPhone 3GS exclusives. That's great. But Ngmoco do not speak for every iPhone developer, and one day, there will be iPhone 3GS exclusives.
Gizmodo do a good job breaking this down, looking at the iPhone's new graphics chip - Samsung's PowerVR SGX GPU core - as a sign that one day, when the more powerful handset is also the most numerous handset, developers will naturally catering for the device that allows for faster, better-looking games.
Of course, this won't happen soon. There are already 40,000,000+ iPhone and iPod Touch devices out there, and it'll be a long, long time until the 3GS (and its successors) begin to outnumber them. To ignore that install base would be a little nutty. But one day - just like we see with PC hardware - they will.
In the meantime, though, we'll probably start to see more and more developers taking advantage of the 3GS' power by "sliding" their games, allowing those with a 3GS to enjoy better graphics, a faster framerate, etc.