prohibit double edit

This module prohibits a second person editing the
same node which a first person is already editing.

It displays a warning message if two people try to
edit a node at the same time.

If the first person starts editing it will set a bit
and ping it by ajax every 3 minutes.
This bit is then released on save or page blur.

If the browser crahes (not able to send onBlur release),
the lock will time out after ten mintues.

This module is realy light weight. If you looking for more powerfull stuff, have a look at:
Content locking