A graceful restart lets those processes be used again.
This entire problem seems to be going faster now that we've added two new servers, so I'm guessing it has something to do with a bug in mod_backhand.
To temporarily work around the problem I now have kenny's apache gracefully restarting every half hour. This should be often enough. I'll watch it.
We really need to solve this problem, though. Dormando's looking into it more.