Natal is just a high resolution camera...  
I think, though, that this is simply not true.  It's a high resolution camera, combined with an infrared projector and some sensor of some kind, I think.  It also has a mic on it (not sure if the Eye Toy has a mic . . . never used it)
And the software, of course, can make an enormous difference because it's not just a single piece of retail software.  We're talking about the APIs and libraries or whatever (I'm not a computer programmer so shouldn't throw terms like that around, especially when I'm talking to a programmer) that determine the device's potential functionality for every game.  
Even looking at the hardware by itself, it is probably not a lot like the Eye Toy.  But the hardware/software combo pretty clearly set it apart.  I mean, maybe it'll be stupid (it's clearly Wii envy, and most of what the Wii does is pretty awful), but I suspect it's not what you think it is.