Hi Matt,

>panic: startup: cannot allocate VM for buffers
>Stopped in at 0x230154:   lwz r0, r1, 0x14
>db>

This is the same problem I've had on my iMac. The problem occurs when the
kernel tries to allocate virtual memory for the systems buffer cache. It
appears that on machines with lots of ram installed, the kernel is running
out of *something* (possibly tables entries - I'm not sure exactly - I
didn't delve deep enough) and it fails what should be an acceptable
allocation request.  You can work around the issue by adding an "options
bufpages=xxx" entry to the kernel config to specify a smaller buffer cache.
The MD version of the kernel already has such an option in its config
(options bufpages=800) which is why it doesn't have the problem.

Regards,
Louis.