For the record, in case anyone else happens to encounter this -- it
was pointed out to me by a helpful individual off-list that this is
actually a known problem when running binaries mmaped out of AFS,
where my xmonad binary happens to reside. I've changed my xsession
script to run it out of a local filesystem instead and am no longer
seeing this behavior.
Can you give me any more information about this? Simply running executables out of AFS does not have any known issues; if it did, Carnegie Mellon University (my previous employer) would have run headlong into it long since, and it would have been fixed by now.