Khmer for Macintosh
An excellent summary on Khmer fonts for Macs, from "The Phnom Penh", now in Bangkok.
Khmer typography is in a bit of flux at the moment. Most people are
still using older encodings and fonts (ABC Zerospace, Limon, etc),
but it's clear that the way forward is Unicode (notably the KhmerOS
fonts).
What this means is that you'll need to install all the fonts you can
find. Luckily, PC fonts are generally usable on Macs. I've got the
KhmerOS, ABC, and Limon fonts on my computer, and they work pretty well.
The next problem is input methods. You could always use the character
palette for input, but vita is just too brevis. Some keyboard layouts
are available, but I don't know how well they work with OS X. (I was
never producing Khmer text, but always using text others had entered.)
I know that they were working on a keyboard
layout for OS X, and were looking for someone to test it, so you
might write their mailing list and see how that's coming along.