I have created some flac files with grip using the following command line
to tag the files with vorbis meta-data.

-V -8 -T artist=%A -T album=%d -T title=%n -T tracknumber=%t -T date=%y -T
performer=%a -T genre=%G -T description=tgnb -o %m %w

The resulting files are playable in rhythmbox 0.5.1 but the tags are not read.

An example flac file can be found here:

http://www.n0t.net/sample.flac