Hibernating while a tomb is open is bad: it means that your key is written to disk.
We should close every tomb before hibernating.
This can be done using /etc/pm/sleep.d/99-tomb script which does this.

Also, if closing fails (because of pid locking it) we should choose between slamming and preventing hibernation.
Slamming is easier, but could be unintuitive for the user to "hibernate" and not finding his applications open on reboot.
Preventing hibernation seems to be possible, according to this post: http://askubuntu.com/questions/28328/how-can-i-disable-hibernate-completely-in-kubuntu