In the recent issue of Classical Fighting Arts #21, there is a very interesting interview with Sensei Pat Nakata. His instructor was Chosin Chibana, a direct student of Itosu.
The article talks about Itosu's purpose in creating the Pinan, and also the 3 levels of kata application and the moves that were only taught to top students (they weren't "hidden" or secret per se, just more devastating targeting).
I believe they sell the magazine at Barnes and Noble bookstores now.