When you use Linux you are simply using the iDevices regular USB access mode that it uses to get files on the device in the first place. Older iDevices were less protected and that is why you can see the files, newer ones are a bit more tricky in the sense they run a weird operating system that fights back against linux computers when you try to get into them. Hooking the iDevice up to a computer is like using it as an external hard drive, a really expensive one at that too.
Absolutely, and I'm in no way trying to suggest that this is a 'good' thing - simply that it can be done, and I'm not surprised that Apple took measures to stop Linux users copying music FROM an iPod, even though they claim to have abandoned DRM. I have migrated a mountain of iTunes purchases to my T51, the catch being that the 'parent' copy of iTunes and attached store are both long gone from my hard drive. Sure, I had to do some conversion, but its far better than having older tunes trapped on your iDevice. Apple will allow you to download older purchases again if they dont appear in your current iTunes library, but thats not the same thing as giving you *everything* you had on an old iPod.
Anyhoo, I've dragged us off-topic : I would love to hear the difference in SQ (if any) between the CLAS/HP-P1 solution and hooking an iPad to a USB DAC. I seem to recall that Skylab and headphoneaddict both had very good things to say about the iPad a music transport with the camera kit, but a lot of the initial buzz seems to have died off.
