Receptional - 6:35 pm on Apr 18, 2010 (gmt 0)
I add something like &source=twitter onto the URL before I shorten it. Then even if the landing page gets reengineered and put through adifferent shortener, I can see the traffic that came as a result of the twitter links.
This can then be tracked as a campaign or series of campaigns in Yahoo Web Analytics. I expect GA can do similar?