I'm putting this up here since I couldn't find anything about it on the web, in the hope that google will index it and save some future person the hassle that I just went through. Several people describe a problem with the keyboard mapping for the Mac NX client (I'm using version 2.1.mumble, the most recent version as of this writing).
The mapping isn't just odd (like QWERTY → AZERTY) but completely and totally fuxored (pressing "r" yields a string of 15 l's). Nobody seems to have solved this issue, and nor have I, but I have discovered that it only happens when using KDE as your remote desktop. I like KDE, but I'm willing to use Gnome if it means that NX works, which it does.
So the lesson: use Gnome instead of KDE and your keymappings will work in NX.