I just decided to try this on a whim, as NiN's latest album (the Slip) can be downloaded in 24bit, 96000 wav if you want (and I did...) (and it's free, if you haven't heard)
I encoded these WAV files to FLAC, leaving the sample/bitrate untouched, tagged it, replaygained it, added album art... and rockbox plays it WITHOUT ISSUE.
It takes a while to fill the buffers, and I have cut my max playlist size to 2k and max directory size to 200... but I'm still VERY impressed. I'm not sure if those measures were required, but I don't need them any higher anyways. When you can play audio at over twice the Nyquist... just wow. That's on a portable player.
Question though: On a Sansa e200, do we know what actually comes out of the hardware once rockbox is done with it?
EDIT: Whoops, about halfway through the song something Bad happened - the playback thread just stopped, buffers quenched, but memory still allocated. I had to hard-shutdown (hold the button a long time) - this has happened to me occasionally anyways, not sure if it's isolated.