Graham English scripts on top on bit.ly:

A URL shortener service called Bit.ly launched today. It has some great features that convinced me to switch from using Metamark to shorten my URLs. Since I have a slick script that allows me to use the Metamark API from Quicksilver, I had to modify the code to make it work with Bit.ly.

Copy the following script, name it what you want, and save it in your home folder. When you want to shorten a URL in Safari, run the script from Quicksilver and it will put the shortened URL on your clipboard, ready for pasting.