Well, it turns out you can't sandbox Python at the language level. So now each user's code is run (a) in a separate process (b) in a chroot jail (c) as user nobody (d) with umask set to 0 (e) with a time limit of 300ms.
Have you experimented with using linux's cgroups for real security? (e.g. you can white/black-list specific syscalls, set time & memory limits etc, all at the kernel level)
