As much as it hurts to say, Rey Mysterio is most likely done with the WWE. There has been several rumors and reports on the situation. He had some issues with his current contract with the WWE and without his consent the WWE extended his current contract without his consent. They were able to do this via a unnamed clause in his contract.
The 39 year old has spent a hefty amount of his career with the WWE. He's appeared in ECW, and also spent some time in the WCW where he wrestled unmasked for quite sometime. The big thing with Rey Mysterio is of course the many injures he has suffered since joining the WWE.
His injures were the cause behind his contract being extended without him exactly wanting it to be extended. The "added time" or whatever you want to call it as been a staple of WWE contracts. It is used as a way to get more time out of a popular wrestler if he was injured for a big part of his career.
The promotion that has shown interest in Rey is the Spanish wrestling federation AAA. They have all but guaranteed He will be signing with them once his deal is up. This would be a major coup for AAA because they will be debuting soon in the US for the first time ever.
Recently released Alberto Del Rio has also been rumored to be appearing at an upcoming AAA PPV. So AAA can land two dominant Mexican superstars in one swoop.
Losing Rey Mysterio is a big loss for the WWE. Only time will tell what kind of impact it will have on the company. After all he was booed the last time he was seen (during the Royal Rumble PPV in January) so maybe it is a good thing that he may finally be moving on.