I recently got to hold and briefly try the D-100 at Photoshop World, San Diego. Very nice. I now have a D-1. The D100 it's like a toy in comparison. Although, I'm sure the picture quality will be right up there and then some. The D-100... Its very light. And small in comparison to a D-1.
Its got a different metering system and Nikon says it does not have the seals and build that the D-1s have. After you pick it up it is obvious. Still 6 megs - got have a nice image. But again a USB port for down load. The firewire on D1 is fast.
But, you can't beat the price difference. Its costs, vs professional build, and speed down load and FPS and image yet to be determined.
But again truly its not as tough built as the D-1 - not even close. it appears to me if you drop it it will break; whereas the D-1 is a tank.
I'm not advocating droping either. but if you plan on using it - the D1 will hold up for sure.