It IS a secret, and no one outside of Adobe knows. (In fact, it appears that plenty of people inside Adobe don't know, either.)
That said, Adobe has said that they are aiming for a 24-month cycle, with an annual update between. So CS5 came out in April 2010, and CS5.5 came out almost exactly one year later. Investors love to say that "past performance is no guarantee of future results," but I think there's at least a fighting chance that we'll see CS6 before, say, summer.
A better question is, "what is going to be in CS6?!" That's easy to answer: super cool features! (But will they be the "right" super cool features?)