UPDATE: Negotiations are reportedly ongoing still between Sting and WWE. The word going around is that a decision must be made and finalized within the next two weeks if Sting is going to join the company. It's possible the deal has already been reached (see Rob Feinstein's comments below) but we don't have that confirmed as of this writing.
Several factors are being decided at the moment, including scheduling and what Sting is and isn't willing to do creatively. While nothing is confirmed, and Sting could end up working with TNA again, the impression has been given that if Sting were to have a run with WWE, he realizes that now is the time.
Another note going around is that if Sting were to sign with WWE, he would not be competing in a match against The Undertaker at WrestleMania XXX as has been rumored in the past. However, this does not mean that Sting won't appear in some sort of cameo or vignette during the WrestleMania XXX broadcast.
As of Monday, it is believed that Sting had still not signed a contract with WWE. For whatever it's worth, however, Rob Feinstein of RF Video noted on his Facebook account that he heard Sting had in fact signed a deal with WWE at some point in the last 24 hours.ORIGINAL: Rob Feinstein of RFVideo has posted the following on his Facebook about Sting signing with WWE…
"I was just informed Sting has signed a deal with WWE in the last 24 hours. I still think personally he will show up after Mania since it seems like Undertaker vs Brock is what they are aiming for. Reminds me when everyone thought Brock would debut in Miami but they waited for Raw."