So, today I installed RockBox on my iRiver IHP-140.
It's not as pretty as the original firmware (which, incidentally, I can still get to because, well, the RockBox guys are pretty smart), but it has two feature that I always wished for that the original firmware never had–1) the ability to use .m3u playlists that also work under mpd (that is, ones that use forward slashes, as $DEITY intended), and 2) the ability to create playlists on the fly by queueing up tracks interactively.
I would seriously recommend it to anyone who has one of these players, and once the iPod port is to a reasonable point, I'd push people to use it on those too–you get access to actual free formats, like OGG and FLAC, instead of being tied to MP3 and AAC.