Umm, there are some released, Sonic Rivals, LocoRoco, Ratchet and Clank multiplayer demo (that is not exactly the same as the final level of the retail game), the GPS stuff (dont remember the name) and a few more.
I've found that a number of retail games or demos have beta material or versions lurking somewhere in their content. Ghost Recon Advanced Warfighter 2 can be converted into a pre-release version by changing some variables in the game.txt file. The ATV Offroad Fury Pro demo also has beta material not in the final game, which can be accessed via one of the config files. And Star Trek: Tactical Assault has a load of unused ships and levels.
I'm surprised there hasn't been any PSP launch games that have had their pre-release versions leaked (Review copies, beta code)
You're not looking hard enough: http://www.assemblergames.com/forums/showthread.php?33484-Killzone-Liberation-preview-0-01
And it's not as though PSP beta versions are readily available, especially for things with substantial differences, not that they dont exist http://ptoponline.com/?p=884
