Dana White : Brock can retire, but he's under contract

At the UFC 141 post-fight press conference Dana White was asked about Brock Lesnar's retirement from MMA following his loss to Alistair Overem.
The UFC's President response was simple: "When he retires, he retires under contract," White said.
What White most likely means is that Brock may or may not actually retire, but if he does, he still has a UFC contract to honor, therefore would be unable to compete in any athletic event outside the UFC, and more in particular the WWE. It has been rumored that Vince MacMahon would love to have Lesnar back and even tried to negotiate an appearance at this past Wrestlemania.
The UFC is smart, and if Brock Lesnar is indeed retired,  you won't see him in any arena until his UFC contract is expired.

 source: The Underground