As noted earlier, Rey Mysterio has been dealing with lingering knee issues, and recently feared that he had torn the ACL in his good knee. He was told by Dr. James Andrews that he wouldn't require surgery and would be able to wrestle with the help of a special brace.
On the Wrestling Observer Radio show this past weekend, Dave Meltzer noted that while Mysterio is "officially" slated to return in June, he may not return to action in the company at all.
"The official company line on Rey Mysterio is that he will be back, hopefully, in June... after rehabbing the knee," said Meltzer (props to Sharon Glencross of The Bleacher Report for the transcription). "But he may not be back at all. And it's not retirement, but there's something going on... there was stuff going on that they were working on perhaps a release, but they want merchandising rights... but officially he'll back in June."
For what it's worth, Mysterio is listed on the "events" page on WWE.com for the Monday Night RAW following WrestleMania 29 on April 8th from the Izod Center in East Rutherford, NJ. He is also listed for the RAW WrestleMania Revenge Tour in the U.K. in April. However, it may simply be a case of the listings being outdated.