I had bought a set of the variable pitch props fromHobby Lobby years ago, but I chickened out and never used them.eventually I gave them away to someone else.
I think the set I had wasn't in flight adjustable though. You dialed in the pitch you wanted on the ground when the engine wasn't running.

They were discussing them here too: http://www.rcgroups.com/forums/showthread.php?t=992229