Answer is most completely stated here:
<a href="http://help.twitter.com/forums/10713/entries/15348" rel="nofollow">http://help.twitter.com/forums/10713/entries/15348</a><p>under <i>Your user name or email address is associated with a deleted account</i><p>The long and the short of it: they'll delete deleted accounts after 6 months, or immediately if the owner explicitly asks them to, at which point they're freed up.  They <i>do not</i> speed up this process though, so don't bother asking if you don't own the one you want access to.