Found this article about it:
tr.im is no longer accepting URL shortening requests via its website. May we respectfully suggest that you choose one of the many other wonderful alternatives available.
Please understand that this does not affect any software that has tr.im available within it. tr.im's API is available, and redirections are working normally.
Oooh, sounds like some political intrigue, hand wringing, and possible hurt feelings (not twitter's favorite child?) I know that I'm not the only one who used tr.im beyond twitter posts, though....
I know about tinyurl and bit.ly, but I thought I'd check out other options. I found this review of URL shorteners. Probably more info than you'd ever want about them, but there ya go.