As of last weekend, Sting had not officially signed a deal with WWE. Word is that his contract is still being worked on and that the deal is nearly finished. As previously reported, the merchandising part of his deal is already being worked on.
Sting is still not expected to be a part of WrestleMania 30, and will likely be held out of in ring competition until WrestleMania 31. The idea is that WWE will use his "in ring WWE debut" as a selling point of the event.