This code starts:

  0:   8b 55 04                  mov    0x4(%ebp),%edx
  3:   83 c1 08                  add    $0x8,%ecx

and as %ebp is 01000000, this oopses.
It looks very much like a single-bit memory error (as has already been
suggested as a possibility).

Oh my. So, I ran memcheck again for a few hours, and it checked out fine. 
Just in case, though, I bought a replacement stick of ram. Well, the 
oopses went away, so it must have been the ram.

Sigh. Sorry about that, everyone. I suppose the raid operations are 
particularly memory intensive. Anyway, thanks a ton for all the help.

Will