I don't see this.  I do see occasional disk activity from periodic
  syncs and cron activity.  How much memory do you have?  Maybe you're
  constantly paging lightly.

No, something else is going on.  I don't run cron, and I've got plenty of
memory.  I can run as many processes as I like without exercising the disk,
but as soon as I start up the X server, I get periodic activity.  I'll see
if I can figure out what it is.