The Deja Vu status icon remains in the menu bar, and the 'View Log' button in Deja Vu displays "working", even though no backup is actually being performed.
This usually happens when a backup is terminated abnormally--as the result of a power failure, or when the computer is shutdown or restarted while a backup is in progress.
Restart your Mac if you haven't done so recently and then try to perform a manual backup (add a new folder temporarily if necessary). If you're able to do this, things should straighten themselves out once the backup finishes.
If you're unable to do the above for some reason, enter the following command in the Terminal:
On Jaguar or Panther:
sudo rm "/etc/periodic/daily/.lastbackup"
(enter admin password when prompted)
Tiger users should enter this command instead:
sudo rm "/Library/Application Support/Deja Vu/.daily/.lastbackup"
NOTE: This problem should be fixed as of Deja Vu 3.2. (Just open Deja Vu and it should reset itself.)