Delicious creator Joshua Schachter has a good post on the downside of URL shorteners. I think his prescription for addressing them as a problem are right:
One important conclusion is that services providing transit (or at least require a shortening service) should at least log all redirects, in case the shortening services disappear. If the data is as important as everyone seems to think, they should own it. And websites that generate very long URLs, such as map sites, could provide their own shortening services. Or, better yet, take steps to keep the URLs from growing monstrous in the first place.
Update: Looks like there’s a larger conversation about URL shorteners going on. Kellan Elliott-McCrea posts about how sites that provide their own shorter alternative URLs should annotate them, and about Flickr providing its own short URLs.
In 2007 there was a big discussion of the problems with URL shorteners. Here’s my post from back then.