Hi tmw,
We're still working on investigating this. I've determined that Mac OS has a problem dealing with low memory situations when only one processor core is available. The same problem occurs on physical Mac systems which are configured to only use a single core and ~2 GBytes of RAM (boot-args="cpus=1 maxmem=1900", for instance).
Until the underlying problem is solved, you should probably run all your Mac OS 10.8+ VMs with at least two processor cores allocated to them.
Cheers,
--
Darius