DivX, PS3 and Xbox 360

How is it that DivX files that play on the Xbox 360 don't play on the PS3? Isn't DivX DivX?

I converted some files using ffmpegX, and they play fine on the Xbox 360. The PS3 says it's an unsupported format.

Looks like both platforms play WMV (surprisingly enough) but I can't find a WMV converter for the Mac.

Is there a decent format that is compatible with both systems, that has a converter readily available?