Yes, FLAC is gapless. I've also tried Ogg and that works great too.
The only format I've never tried is WMA (because I've got no Wma files in my collection), so I can't say anything about it being gapless or not.
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Riocar 80gig S/N : 010101580 red
Riocar 80gig (010102106) - backup