Need to do a Time Machine backup right now? Locate your Time Machine disk in the Finder, right click on the disk, and select Back Up Now from the contextual menu.
[robg adds: You can also control-click on the Time Machine icon in the Dock, where you'll find the same option, as well as the ability to browse other Time Machine disks and open Time Machine's preferences.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20071030060847886