Just thought I'd update this thread in case other developers are looking to get deleted usernames released for their apps too.
Twitter got around to answering my support request however they are not going to release the deleted username to me, and state: "Due to high ticket volume, Twitter Support is no longer releasing inactive user names unless in cases of trademark or copyright violation. We are working on releasing all inactive user names in the future, however, we will no longer manually release them on an individual basis." It's a shame, as the username was perfect for an application I'm developing, however I've had to register something else instead now which isn't as sexy, but it'll do. I guess if/when Twitter release all inactive userames at some point, I might have another chance.
