When Apple's Backup.app runs a scheduled backup plan, it pops a dialog with a 120 second timeout. You could just ignore the window and Backup would function normally, but I get annoyed by it just sitting around doing nothing for two minutes.
The following shell command lets you set the timeout to however many seconds you prefer:
defaults write com.apple.Backup 'Backup Timer' -int '1'
[robg adds: I haven't tested this one...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20051230013123369