It doesn't work for me either since upgrading to the newest DIVX, but the new DiVX player has subtitle ability in preferences and that works just fine.
I have Mac 10.6.8 fully updated
QuickTime 7 pro and player
Perian 1.2.3 installed in system preferences and appears to work (I can enable and disable options)
Movie is an AVI file, sub titles are .srt files, named the same as the AVI (except the .srt obviously) and in the same directory.
I wasn't getting any audio in QT7 either until upgrading to the newest Divx, It came with a player and the subs work perfectly in there.