I've had this happen twice now, running Andrew's "not quite -mm2" patch.

Symptoms: After about 20-30 minutes uptime, a running gkrellm shows system mode
suddenly shoot up to 99-100%, and the keyboard dies.  Oddly enough, a USB mouse
continued working, and the X server was still quite responsive (I was able to
close Firefox by opening a menu with the mouse and selecting 'quit', for
example).

alt-sysrq-foo still worked, but ctl-alt-N to switch virtual consoles didn't.
sysrq-t produced a trace with nothing obviously odd - klogd, syslog, and the
disk were all working.

Nothing interesting in the syslog - no oops, bug, etc..

Another odd data point (I didn't notice if this part happened the first time):
gkrellm reported that link ppp0 had inbound packets on the modem port of a
Xircom ethernet/modem combo card.  At the rate of 3.5M/second - a neat trick
for a 56K modem.  When I unplugged the RJ-11, gkrellm *kept* reporting the
inbound traffic.  When I ejected the card, *then* the ppp0 (and the alleged
inbound packets) stopped - but still sitting at 99% system and no keyboard.

This ring any bells?  Any suggestions for instrumentation to help debug this?