Have you ever had Time Machine get stuck--really stuck--on a backup? So stuck that clicking Stop Backing Up does nothing, so stuck that moving the On/Off slider in the Time Machine System Preferences panel to Off does nothing? If so, then this hint may help you get out of that "stucky" situation.
Open Terminal, and run these two commands:
sudo service com.apple.metadata.mds stop
sudo service com.apple.metadata.mds start
Press Return after each line, and wait for the prompt to come back each time. What this does is stop and restart the process that controls Time Machine. When that happens, that should force the currently-stuck Time Machine backup to simply exit gracefully.
Note that if you use these commands in 10.6, you'll get a message about their usage being deprecated. They still work, so it's nothing to worry about. (You should be using launchd to control these services in 10.6, but the above commands work just fine.)
Thanks to Mac OS X Hints reader Frett for pointing this one out.