As discussed several times before, including http://bugs.musicbrainz.org/ticket/4776 (and more recently, http://forums.musicbrainz.org/viewtopic.php?pid=16882) Picard writes sort frames for ID3v2.3 to some user-defined frames XSOP/XSOT/XSOA. Virtually nothing can understand this. There are no official frames for writing this in 2.3 however many programs do understand the 2.4 frames being written (TSOP, TSOT, TSOA).

Informal list of apps that understand this non-standard frame (please help add to this)
mp3tag
foobar2k
SlimServer/SqueezeCenter
iTunes (not sure which versions)

Propose here that we continue to write XSOP/XSOT/XSOA but also write TSOP/TSOT/TSOA additionally to this.