Like Facebook, it is clearly a move into penetrating the real-time web and its culture of sharing / clicking. It is also a way to determine trending and velocity while delivering analytics to publishers, users and brands.
And as it pertains to brands, it will be interesting to watch whether brand plays a role with URL shorteners... or whether success is a function of in-application / in-site availability (for instance, all things being equal, FB.me should win on Facebook). The real battle will be for off-domain activity - Google's toolbar is a powerful tool here and Facebook Connect could / should play a key role as well. As TechCrunch notes, off-site scale matters:
Up until now, bit.ly has moved quickly to become the standard shortener. But the sheer volume of short links which both Facebook and Google can produce could soon overwhelm the number of bit.ly links. It’s the data behind the links, however, which is valuable. Whoever can gather the most unified view of all shortened links will end up winning.
How long before Twitter announces their shortener?