Idea taken from doctype.com
When a question is asked an auto generated short URL (perhaps using a special "short domain" purchased for just this purpose) provided for use in twitter, or if passing along in SMS format.
On 10/19/2011 a blog post on Stackexchange.com for podcast #23 provided notice that StackExchange.com has the amazingly short URL s.tk so it would seem that the StackExchange.com sees value in having a "short URL", using s.tk as the domain you could access a question on stackexchange.com such as s.tk/joel to access Joel Spolsky's user page on StackExchange.com
It would seem that the top sites (such as StackOverflow.com, SuperUser.com, ServerFault.com, etc.) would also benefit from a short URL as well as an "auto generated" short URL for questions on top SE sites.