Good news! I finally got a chance to play the 3DS and see how it works.
Specifically, I went to a Best Buy, which had a demo 3DS for people to test out. The 3DS was about three feet off the ground, so I had to kneel on the ground to get close to it, then hunch over so I could see the screen. Something tells me that's not the normal body posture used to play the 3DS.
The game that the system was playing was Dead or Alive: Dimensions. It's a fighting game, and it must be the easiest fighting game ever, because I won every round, even though I had no idea what any of the buttons did.
So, what is the 3D in the 3DS like?
It's like the lenticular picture on the cover on the Legend of Zelda: Majora's Mask cartridge. You know, those pictures where you turn the piece of paper, and the picture changes? And sometimes those pictures will kinda look like they're in 3D, with the picture coming out at you? Only the effect is totally spoiled because you can sometimes you see the after image.
That's what the 3DS 3D looks like. It's apparently the exact same 3D technology as those pictures, which works based off using two pictures. They call it "stereoscopic".
The 3D effect works okay. Dead or Alive is not a good example of a 3D game, because it's a 2D fighter. What makes it 3D is that the fighters, fighting in 2D in the foreground, are about a centimeter closer to you than the non-moving background. The characters never moved closer or farther away from the screen or the background. So, the game basically just ignored the fact that the graphics had depth.
The other 3D effect was the "you just won the fight" text. That didn't work so well, because the after image was blatantly obvious, and the effect was totally ruined. Is "after image" the right term? In any case, you could see the text on the background of the screen and the popped-out version of the screen at the same time. It didn't work to have both at the same time.
All in all, I'm not really sold on the 3D effect. I'm sure it could be interesting, but since it's only about a centimeter long, I don't really see how it could majorly affect gameplay. With Dead or Alive, there is basically no difference between 3D and 2D mode, but like I said, the game is a 2D fighter, so it's not the best game to judge things by. Maybe I'll wait for a Super Mario 3DS demo.