I personally don't really buy that. I understand the "iPod touch is for games" idea, but to be honest, it's hardly true that people who want or need smartphones necessarily don't like video games. There seem to be plenty of people playing these games on the iPhone. I had a DS Lite before, and I loved it, but I've been playing games predominantly on my phone.
As for pushing the hardware, you make a point, although I think it may not be a deciding factor. In the console world, the hardware is fixed for a long time, and so developers really have to get creative to squeeze performance out of the games. So when you look at a DS Lite game that was written 4-5 or even more years after the DS came out, it's much better than any launch game.
With the iPhone and iPod, the hardware is being updated regularly, so coders can just produce "average" code in terms of efficiency and demand that the hardware suck it up.
Already there are definitely games that had choppiness issues even on the iPod touch 2nd gen that run smoothly without modification on the iPhone 3GS. So I think there're already games like this. Could that game have been optimized to run on the earlier hardware? Sure, probably. In some cases, definitely (one example of a choppy game is Sonic the Hedgehog... I mean, please. This is Sega Genesis Sonic the Hedgehog. There's no reason it can't run at a high framerate on
any generation of iPhone / iPod touch hardware

It obviously isn't well optimized).
But I think the update cycle will lessen the extent to which developers find optimization valuable, and so there I'm pretty sure there will be games that run unacceptably on the lesser hardware and shine on the newer hardware, and that it will happen fairly quickly.