I very well be wrong about this (i.e. someone please confirm), but doesn't the ipod touch have a 400mhz processor while the iphone has 600mhz?
While many applications could work for both, if this is true it could mean that some, like spore, don't work as well or at all on the ipod touch.
However, with the exception of powerful games where they strain the iphone's capabilities, I think most developers will try to make their devices work on the ipod touch because it won't take much and will open up a large market.