Actually, the downloader utilities from eMusic and Amazon pretty much inject those things directly into iTunes.
From what I see, there's no classical filesystem on Apple iPad - just per application resources - so it won't work easily.
Obviously, there are some fixes to this particular problem. However, keep in mind, that I just wanted to give few examples of possible confusions.
Without the actual device, it's hard to give more specific examples of complains.
No need to go into details. From the hackerish perspective, the internals of iPad might be nothing but BSD/Mach combined with touch screen.