You can achieve close to proper frame rates by sticking to this method:

when you set the fps of your movie to 15, 16, 17, 18, 19 or 20 it will playback on the Mac at around 15.6fps.
when you set the fps anywhere between 21-30 it will playback at 20fps.
set it between 31 and 59 it will playback at 30fps.
anything higher will playback at around 61fps.

Some say the playback inconsistencies have to do with the PC using Active X and the Mac doesn't.
Who knows what the real answer is.
I usually stick with 21 fps for all my projects-that way it plays at 20 on the Mac and 21 on the PC

For now you should always test your .swf files with the moock fps MC.
http://www.moock.org/webdesign/flash...ter/index.html

I have found it to be invaluable for testing on the Mac.