There's a DSi game out in Japan that might give an indication of how the 3D will work on the 3DS. It's called 3D hidden picture. It uses the cameras as motion sensors (something that has been hinted will be a 3DS feature) to alter the picture based on console angle.
http://www.youtube.com/watch?v=h5QSclrIdlE
There's also an iPhone game that uses accelerometers to achieve the same effect.
Personally, I don't see the big appeal of 3D. I don't give half a damn about it. I just want a more powerful Nintendo handheld, hopefully with at least double the current machine's resolution per screen.
At any rate, Nintendo says there will be playable 3DSs at E3.