I've found that it doesn't like to shorten certain things, though I haven't quite nailed down what "certain things" are. I tried linking a recent blog post I'd done, which is at least 30 characters (along the lines of http://website/really-neat-post-title), and when I pasted or typed it into the tweet, it wouldn't autoshorten and would consume those characters.
I find myself using goo.gl a distressing amount of time on Twitter.
The interesting thing about that is that twitter will still generally redirect it.
As an experiment some time ago I grabbed a random sampling of URLs from twitter's limited firehose - and found that on average they spread the user's trail across no fewer than 4 redirects before the final destination was reached. Some as many as 7 or 8.
I've stopped posting URLs to twitter, out of respect for my followers.
something as simple as 邗諾
becomes %E9%82%97%E8%AB%BE , 18 characters!
Still, as other mentioned it's a neat idea that if widespread would make all kinds of encoding mistakes pop up :)