About hi-rez files on a Rockboxed iPod: I have an iPod Video 5.5 gen (originally 30GB) and an iPod Mini 2nd gen (originally 4GB). Both have been upgraded to 128GB solid state via CF cards (the former using Tarkan's board). I *play* 24 bit/96kHz files on them all the time in WAV format under Rockbox, but they are *downconverted* to a lower sampling rate, probably equal to the Frequency parameter described above. I also tried high-rez ALAC but got skipping as well. When you go with an uncompressed format like WAV, Rockbox on an iPod keeps up. In fact, I can also use easily 7 bands of EQ and there's no skipping heard (although the interface slows down a lot).
Now, about the playback itself: I checked the frequency response of the playback via a simple log sine sweep out of the iPod Video line out for 44kHz, 48kHz, and 96kHz uncompressed WAV files. What I found was nearly flat frequency response with 44.1kHz files, a slight drop-off in frequency response (say above 15kHz) with 96kHz files, and a more-severe drop off in frequency response for 48kHz files, all when playing back at 44.1kHz. What that says to me is that Rockbox uses very simple interpolation to do the downconversion. I didn't check the filter math or keep careful track of the dB drops, but I suspect it is linear interpolation.
Bottom line: You can load up 96kHz WAV files on a Rockboxed iPod and they will play. There will be a very slight loss of high frequencies when you do, and you can decide for yourself by listening whether it is noticeable. For me, the better mix on the 96kHz files makes them preferable to the older 44.1kHz files I have for those albums that I have in both formats (mostly Steely Dan/Donald Fagen).