According to Major Nelson it appears that Microsoft is currently in the process of deleting old Gamertags out of their memory banks. This means that these old dead Gamertags will be able to be used again by those who are interested in using them of course. Now what exactly makes a Gamertag “dead” has not been detailed yet but it is suggested that any account that was linked to an original Xbox will be removed.
Of course this also means that you can now spend the 800 MSP to change your gamertag to whatever you want it to be if the feeling grabs you to try and get a name that you have always wanted, but for some reason was always gone despite all of your efforts to track it down.