After hearing the latest news it now seems certain Christian will return to the WWE. His contract has expired and his profile has been removed from TNA's website.
If Cage were to stay at TNA he would be the leader of TNA's frontline and the biggest face in the company after being attacked by the Main Event Mafia recently (as most of you know this was just a way to write out Christian if he were to leave).
The general feeling in TNA is that he had made his mind up months ago and in a recent crisis meeting informed them that he had received an offer from another company and was strongly considering it.
His return to WWE isn't a certainty but since Cage was wrote of our TVs a couple of months ago he hasn't had contact with anyone at TNA and it is extremely likely that he'll debut on smackdown soon.