I've heard a few different reasons for this. One is that the bubbles mechanism needed repair. The other is that the straitjacket was starting to mildew and the fish were eating away at Houdini's face. (I guess the "experts" at Tanked don't think beyond the air date, eh?)
But I'm also hearing this is only temporary. The mannequin will be repaired and put back in the tank. But I actually think the aquarium looks pretty good even without Harry at home. I think the fish might like it better as well.
Thanks to MSW.
UPDATE (2/2/13): Word is Houdini has returned.
UPDATE (6/1/13): So it looks like the Houdini mannequin has been permanently retired. Word from AMA President Neil Patrick Harris is that a new mannequin will be made for the tank by animatronics whiz, Garner L. Holt. This new Houdini will move and will be in handcuffs and a swimsuit to more historically correct.
UPDATE (6/9/15): The Magic Castle bids goodbye to the Houdini aquarium.