Well bad luck you have to wait longer. But while you wait, you might like to read Tim's description of his game on the Double Fine Top secret Game in Development page. and then try and guess from the other information just what the hell ?Psychonauts? is going to be like... I am.
Update: RemiO has reminded me that there is a bunch of info on this game, including a movie, posted on The Scummbar.I'm going to go watch it whilst I sit here, clueless and holding the fort so to speak.