While there are recurring rumors of a iPod SDK from Apple, as of yet there is now way of doing this. Since this is an embedded processor running a non-Apple OS this would require that Apple do a lot of work on a cross-compiler, or create a scripting language for the iPod (not generally something you do on embedded systems).
And I just don't see there being much up-side in it for Apple. What are small developer third-parties going to add to the iPod that would increase sales significantly. And with Apple apparently running at capacity right now...