OK, so GDC is March 11, Valve is going to be at GDC. March 11th is 3/11, which is the state of developement that GLaDOS is in. 3/11 is the day after 3/10. 3/10 is March 10th, also known also Mario day. (Mar10)
 One of  the decoded images shows two people with portal guns. this leads me to believe that Portal 2 is going to be Super Portalio Bros, where you play as Mario and Luigi and you have to use portal guns to go back to the mushroom kingdom.

My theory is correct.