As an Apple consultant, it never fails that when I'm looking at a client's system, as soon as I do a System Update, the Time Machine backup system wants to kick in.
Since turning it off could have dire effects if I forget to turn it on again, I've learned a little trick to give me a one hour maintenance window to work on the machine and not get slowed down by Time Machine hogging the disk I/O.
If Time Machine starts up on you, quickly go to the Time Machine System Preferences panel. If you look to the right of the Options button, you will see a progress indicator. If you click on the small circled X at the right end of the progress bar, you just bought yourself an hour before Time Machins starts another backup.
I tell clients to do this over turning Time Machine off and on all the time. If it isn't always running, you're not really doing backups, and missing the key features it offers.
[robg adds: I don't know how this differs from clicking on the Time Machine icon in the menu bar and selecting Stop Backup, but it's always nice to have alternatives.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=2008122019411278