I think the problem with this thing is, that there is no real partition, where the system and the programmes (calendar, clock, contact etc.) are on. the partition where the system is on seems to have no file system, it just starts with the machine code. So I think everything is packed into one code cluster. Cause of that you wouldnt write a programme for the ipod but a patch, that its os would include a new feature. That would be possible, but I dont think that apple will bring out a SDK therefore.