xautolock 1:2.1-7.1 (i386 binary) in ubuntu lucid

 Xautolock monitors input devices under the X Window System, and launches a
 program of your choice if there is no activity after a user-configurable
 period of time. You can use this to automatically start up a screen locker
 if you have left your computer unattended for some period of time. The
 program launched need not be a screen locker such as xlock.