Brock Lesnar has signed a contract with WWE although full details are not available as of this writing.
As I noted here on WrestlingNewsWorld.com Premium, Vince McMahon wanted him to appear at Wrestlemania 28 but it didn't happen because a deal couldn't be reached. McMahon didn't want to rush things as he wasn't clear on the direction he wanted to go with Lesnar. While I do not have full information on what WWE plans on doing with Cena and Lesnar, there is clearly a plan in place for a major main event match.
For those wondering about Lesnar's UFC contract, I'm told he bought himself out of his UFC deal, however, the deal carries a non-compete clause, preventing him from competing for another Mixed Martial Arts company. The non-compete clause isn't believed to expire until his UFC contract would have run out.