A Simple Process
Under Twitter’s Policy Information there is a section on Inactive Accounts but at this time there is no way to claim a username from an inactive account. As you would expect Trademarks are protected, but you can also claim against impersonation.
Stop Impersonating Me
As the Twitter username that was inactive matched my real name exactly I completed the Impersonation Form. I also explained that the user in question had not used the dormant account for nearly 3 years, on the form.
I may have been helped by the fact I also own the domain SeanClark.com, and was able to include an email address at that domain.
If you have been waiting on Twitter to release a dormant username, the impersonation clause may work for you.