Just noting that this post is from 2009, so may be very outdated information.
Good point. It may be of interest, I wrote this: http://moz.com/blog/url-shortener-owl-li-indexed-in-google about ow.li using 302 redirects, and shortly after the post Hootsuite switched oow.li to 301 redirects.
I'm always with ow.ly !
Many of these services are defunct by now.
One of the biggest factors with URL shortners is you want your links to be alive tomorrow, the day after, etc.
Fly by night URL shortners cause big issues for lots of people/links/301s over the years.
Great! Head over to login and use your credentials there