With versions of OS X 10.4 prior to 10.4.9, it was possible to use not-officially-supported phones with iSync. There are several hints here that show how to do this (it usually involves copying an existing iSync plugin for a supported phone and modifying it slightly to match the unsupported phone). I had happily been syncing my Nokia E60 in this way for many months.
So I was very unpleasantly surprised to notice that after the OS X 10.4.9 upgrade I could no longer sync my unsupported phone. After spending hours trying to make this work, I crossed the pain threshold and bought a set of plugins from novamedia.de, but they did not work.
Finally, after hours of searching, it turns out that Nokia now provides iSync plug-ins for a whole slew of phones, including models that Apple doesn't list as supported. From the Nokia site, you can now get plugins for all these phones:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070406143513710