As previously reported, DivX has been slowly evolving for the Mac...
A port of ffmpeg at divx.jamby.net was the previously available DivX decoder available for Mac OS X. This port did remarkably well, allowing OS X users to play the majority of DivX files (with the proper conversion).
Divx.com has been testing an internal version of the "official" Mac decoder for sometime and has just released an alpha version. This version allows for 6 levels of post-processing filters for better image quality and also runs under Mac OS 9.
Of note, there is a nagging problem with Apple's Quicktime parser for AVI's and as a result, many DivX's with MP3's have sync issues. Divx.jamby.net's port port provides an excellent AVI2Mov utility that is still necessary.
Encoding should be soon available with this interesting comment:
I also expect the ppl will be very suprised when they find out that they can encode to DivX faster on a G4 Mac running OS X then even the fastest PC available today when we are finished with it. The question then of "Faster then Real-time encoding" will not even be an issue from that point on.