I've just installed the i386 snapshot on a machine that was working
   fine for some time. The kernel can't manage to find a number of
   important devices -- including the ethernet board -- and has
   hallucinations about others including an mcd device.

Is your ethernet board at one of the addresses specified in the config
file?  It doesn't sound to me like you even checked...

There's very little I can do about the mcd probe falsely recognizing
devices occasionally; the protocol just sucks.

   Thinking that the right thing to do would be to build a new kernel
   with most devices not on the machine stripped out of the kernel (which
   usually cures this sort of madness), I began the process with an
   innocent make depend -- which reliably throws me into ddb with a
   vm_fault.

I can't *possibly* determine what would cause that without at least a
stack trace from DDB.