I get a 'panic: pmap_page_protect !PG_V'

i get this every other month or so, on average.  it always happens at the
same point of time, though:

i boot the machine.  i login.  i run startx.  my `main' xterm starts up.
i run screen......boom.

i've seen this happen about 10 times or so since early last year.

.mrg.