thinkStorygood advice:
"If you leave Twitter, I recommend you DO NOT delete your account. All old tweets @ mentioning you will still exist and if someone grabs that now available username then all tweets @ referencing that username now point to them."
Del 
@thinkStory old usernames are locked afaik. I joined originally in 2008. I deleted my account a couple of years later but when I eventually rejoined I was unable to create a new account with that same name. Not in use, just locked.