Well informed sources with World Wrestling Entertainment do not believe Steve "Sting" Borden has signed a contract. That's not to say Sting can't sign at some point, but it does not appear as though he has at the moment. It appears Sting is still a free agent as sources within TNA aren't aware of him re-signing with the company. For what it's worth, he's still listed on the TNA website roster page.
If Sting does sign with the WWE he may well be a surprise inductee into the WWE Hall Of Fame or perhaps even an opponent for Undertaker, only time will tell, TNA seem to think he may still appear back there.