ive had some decent luck getting twitter names that are considered inactive
use this form :
http://twitter.zendesk.com/requests/new
in the drop down field go to inactive account
and put in the other fields about how it is a name you use online and show some sort of proof that it is of use to you and they will kill the account and it will be open for registration.
im not so sure about how true the inactive for 6 months that was stated in this thread is.
The only names that i could ever get released had last post dates of 2007 anything after that was never responded to and if the account never hada post it was never responded to aswell