Linux and classic Macintosh mix well because neither one tries to be the other. The classic Mac is unstable and inflexible, but has an excellent user interface and a good range of everyday software. Linux is fast and sturdy, but doesn't have a well-integrated user interface or big-brand non-scientific software. But now both platforms are stretching toward the middle: the Macintosh with Mac OS X, which replaces the system software with BSD; Linux with Ximian Gnome, which is a real user interface.
Presumably, the Linux developers as a whole will realize that fancy user interfaces are not optional when dealing with anything but flat ascii, and so one day I'll have an open-source equivalent of Adobe Illustrator(R). But until Linux steals the good parts of the Macintosh, it's quite convenient to use both on the same box -- because of their completely different philosophies.
(It might be argued that Windows(R) failed by trying to be an OS for both servers and desktops -- spreading itself too thin -- but that's another node.)
The factors mrsid mentions above, plus the remarkable excellence of software like Yellow Dog Linux and Mac-on-Linux, are both the cause and effect of this symbiosis.