
Create a new shell entry in GeekTool, and enter the following command: This scans the system log file for the last three entries from backupd, displaying them on the desktop. Use whatever refresh value you'd like; 10 minutes is good enough for me.
I'm also able to monitor remote Time Machine activity from other computers backing up to a local disk connected to my computer. Just use this command in GeekTool: If you've set up passwordless ssh login, you're golden.
[robg adds: I haven't tested this one.]