What a confusing week in the world of link shortening!
You may have heard that tr.im is closing shop by the end of 2009, but they have changed their minds several times. First they said they were closing the service immediately, and turning link forwarding off at the end of 2009. Then tr.im explained how costly it was to operate a URL shortener, and revealed that they had chosen to continue the service (due to the user backlash) while quietly approaching select businesses to buy their assets.
Having gotten no bids in the ballpark they wanted, they are ultimately going another direction: open the service up to the community.
They are releasing the code under the MIT license which ensures that it is available for anyone to use freely. Furthermore, Eric Woodward, an employee of Nambu, pledged that he "will personally guarantee any shortfall in tr.im’s operating expenses, indefinitely." This doesn't sound sustainable, but that's the plan. They will also depend on donations, in exchange for the ability to host the service on your own domain if you donate some money.
The problem with using any URL shortening service is that if a service goes down, so do the links. They all waste away, which is why this is known as link rot. All the tweets, posts, and other content using the links would be worthless, or at least broken. So having a community-owned, non-commercial service allows any volunteer(s) to take the reigns if they so desire.