UPDATE: As of OS X Leopard version 10.5.2, Apple includes a menu bar item to provide this same quick access to Time Machine. Keeping the same sense of simplicity I try to apply to my Dock, I think I’ll keep this out of the menu bar and save on some of that limited screen real estate.
I try to keep my dock as simple as possible, and reserve it only for applications I frequently use. By default, Leopard adds Time Machine to the dock, which I find to be just a bit of showing off what’s new to those unfamiliar with Leopard’s features.
Since Time Machine works behind the scenes to backup your data, you’ll hopefully never have to touch it until you need to recover something, therefore making it pointless to keep in the dock.
I keep an alias to Time Machine.app in my Time Machine drive. That way, if and whenever I ever need to fly through deep space to recover a file, I need only open my Time Machine drive in a Finder window.