This is clearly not a widespread issue, or I imagine it would already be solved, but if you have a huge mp3 collection including some old files encoded with Xingtech's (when there was a Xingtech) Audio Catalyst in VBR (or presumably the early Real implementations of this, as they bought them out), Rockbox doesn't like them (you get a 0:00/0:00 and no file error, I'm guessing because there are no proper VBR headers).
I found a nifty command-line utility, mp3packer, on the HA forums which, while mostly for compression of mp3's, incidentally writes the correct VBR info for proper seeking. Worked like a charm, rendering all files readable.
http://www.hydrogenaudio.org/forums/index.php?showtopic=32379(Apologies to Llorean for not reading the posting guidelines in the Installation forum, thus forcing him to drop the hammer. Lesson learned:)