If you think Time Machine backs up too often (or not often enough) for your liking, navigate into /System » Library » LaunchDaemons. There you'll find a file named com.apple.backupd-auto.plist. Open it in your favorite text editor, and look for this section:
<key>StartInterval</key>
<integer>3600</integer>
Change the 3600 number to some other time interval in seconds, and you'll have changed Time Machine's backup interval.
[robg adds: Did you see that Apple now recommends that you set your machine to manual Time Machine backups if you use Aperture. This might be an easier option, if you set it to some large-enough interval (four hours?) so that it won't go off during your Aperture sessions. Ideally, of course, a coming Aperture and/or 10.5 update will take care of the problem in a more useful manner. I'm not sure what you have to do, if anything, for this change to take effect.]