Update: Fan attacks referee
Brett does have a contract with the WWE, and he has been performing in FCW for about a year.
I'm sure that if this information is correct, we'll find out later tonight on Raw. (By the way, Floyd Mayweather's run in with police earlier today will not affect his hosting gig tonight, AP says.)
So, either they didn't let John Cena know they were doing this, or the man really is a great actor...