A 1280x720 @ 23.976 fps ProRes 422 file is exported from FCPX 10.0.5 (on SL 10.6.8). Said file is then brought into Compressor 3 on a separate system (running FCS 2, OsX 10.6.8). Compressor 3 misreads fps as 15.02 fps.
This file correctly appears to be 23.976 fps in FCP 6.0.6 (on same machine as compressor 3), QTX and QT7. Also when the file is imported into FCP 6 and then exported (all same settings) out to a new ProRes 422 file, Compressor then sees the correct fps.
Well the first thing anyone is going to tell you is to update to 10.0.7 There have been so many fixes in the two updates since 10.0.5 its going to be hard to convince anyone not to try that first.
If your worried about upgrading, don't. I've upgraded through massive projects and it works completely fine. if you want any additional insurance, make a Zip of your current event files, and the entire FCPX program, and you can roll back to it no prob if the update does anything harmful.