I've enabled "ethdebug" in the sonic driver, and was surprised to see that
even though userland was frozen, I could still see incoming packets hitting
the sonic driver if I pinged the Magnum.  Of course there were no echo replies
being sent by the Magnum, so the plot thickens.  Also, I saw periodically...

sonic: receive descriptors exhausted
sonic: receive buffers exhausted

...which I normally don't see without ethdebug enabled when pinging the hung
machine.  Not sure why.  But I'm still leaning towards a possible DMA
corruption problem.  I can think of no other reason why the kernel's IP stack
would no longer reply to echo requests after userland hangs.  This has really
turned out to be one hell of a puzzle!

-Mark