The trojan horse model for the iPod is the current presentation of iTunes, but make no mistake about it, iTunes will generate money for Apple in the future.
At some point, iPod saturation will hit critical mass and the sales will tail off dramatically. As it is, a 40 GB iPod is huge, 99% of the music-listening buying public just don't need that much space, nor ever will. This is why the iPod minis are successful. Enough space, looks cool, slightly less money. Good enuff.
Whenever that critical mass hits, iTunes will presumably have a boatload more songs in the library, let's call it a million songs growing every day. What's Apple getting, 30% per tune sold? And how many tunes DL'd these days? I-don't-know-how-many-millions? iTunes will definitely make money for Apple soon enough. The R&D is already done. The hardware update requirements will be a pittance relatively speaking. Plus, the marketing requirements will lessen because iTunes will remain the best known brand for music downloads.