There was a recent thread on anythingbutipod.com about this (there's actually a MS
rep that happens to post to that board). Many people, myself included, have requested FLAC support. Although he's mentioned that he will send these requests back to the development team, I wouldn't hold my breath on it being implemented.
As for Rockbox, very doubtful given that MS digitally signs their firmware. This is why there's no Rockbox on any of the legecy Zunes, yet. If you really need a FLAC device, you should look elsewhere unfortunately.
EDIT: 1,000th post! Woot!