Looks like Nintendo fans are getting "The Big Tease" by Sonic Team.

Here's a rather rough translation of the japanese text that accompanies the mystery screenshot:

"Unfortunately, we can't reveal our fourth title here. But if you like try and figure it out for yourself in the meantime!"

Just before that is a pixellated drawing with information on PSO ver 2 for the GC.

"Furthermore as an added bonus, the GameCube version will feature two new character classes. Just what kind of character? Find out at the show!"

Update: We seem to have identified the shadow on the right. It appears to be a HUcaseal, a female Android hunter. This class was originally meant to be included in PSO ver1 for the DC, but was sadly cut due to time restraints.

Here's a picture of the concept art, which you should note is a perfect match for the pixellated shadow.