Updated 1/8/10: just use this!
If you need to alter the frequency of Time Machine backups there is a manual way and way using a GUI editor. I have included both methods.
Navigate to the following file: System/Library/LaunchDaemons/com.apple.backup-auto.plist
Edit the line containing the Integer value currently 3600 seconds. If you want to back up every half hour then change this integer to 1800.
Or you can use Terminal
sudo defaults write /System/Library/LaunchDaemons/com.apple.backupd-auto StartInterval -int 1800
Once again the above value is for 30mins to run the Time Machine daemon. So adjust it longer in whole values if you want more time between backups.
The GUI method
Time Machine Scheduler
Looks like a pretty solid app and gives you some functionality even the ability to turn off the Time Machine Daemon for periods of time. The downside is that depending on what settings you chose you may have to turn off Time Machine in the prefs pane and just use this tool instead. So prolly not a great fit for most clients.