Chances are quite high that this is still a work. With the current direction of TNA and their sudden affinity for work/shoots and shoot/works and fooling the IWC, you really cannot believe anything they say, especially via tweets, e-mails, whatever. Maybe it is legitimate and if so I hope he gets a proper send-off on his final iMPACT, God knows the man deserves it. After what he's done in WWF, WCW, and TNA, he is certainly worthy of a proper sendoff.
However I believe this is a work. Either it is part of the plan which will be unveiled shortly, or it is a bargaining strategy on his part to try to get a new deal with TNA. I wouldn't be one bit surprised to see Nash, Sting, Joe, and Pope confront "them" on iMPACT, only to have Nash turn on the other three and re-join nWo 2010 version.