This comes from a Game Informer interview, sent in by reader Eric.

GI: What is the little door on the front of the unit?

Fils-Aime: That is where an SD card goes. That’s storable media. And on
top, are the attachments for the GameCube controllers.

GI: So will SD cards be the primary storage medium?

Fils-Aime: It will be one of the memory mediums, and it’s the only one
that we’ve announced.

Mystery solved...or at least partially solved. Reggie says that the SD cards are "one of the memory mediums". Perhaps there still are some secrets. I also have to throw in a line from Eric's email, it was too good to pass up.

It seems that its just the SD card slot behind the door and not a time machine or a 3D holographic
projector.

Damnit, I was really pulling for that time machine!