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.

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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.