I think br.st! is my new favorite URL shortening site
http://br.st/
not only because of the nice design but especially because of this:
http://br.st/tweetie.aspx
If you follow the instructions on this page you can add a bookmarklet to your iPhone and use br.st shortlinks with tweetie.
Also, these are cool features:
+ add a "+" to the br.st Link to display stats for given URL.
- add a "-" to the br.st Link to opens a preview.