FLAC -> MP3 with MP3FS
I haven't tried this, because I don't have much music on my computer, and I don't have an MP3-only portable player either. But I think it's a brilliant idea.
Get your FLAC on with MP3FS - I don't know if the folks at Xiph.org can live day-in and day-out using only the free Vorbis, FLAC, Speex, and Theora codecs, but the rest of us routinely run into consumer devices that don't recognize and support them. But with a little help from Filesystem in Userspace (FUSE) and MP3FS, you cross one incompatibility off that list. MP3FS lets you mount a directory hierarchy of FLAC audio files and transparently present them as MP3s to software and hardware devices alike. [GNUs]