Reminds me of harvesting ICQ numbers. There was a time when you could search 6-digit ICQ numbers for expired freemail addresses like Hotmail (they deleted your account after a while), register that freemail address and reset your ICQ number password to get a brand "new" 6-digit number. I think this doesn't work anymore, since most freemail hosters don't "free" expired email addresses but keep them locked.

It still works if you find an expired domain name, register the domain name and then do the whole password-reset procedure. Might be cheaper to buy a 6 digit number on eBay though :)