After holding the D3 and D300 for several minutes, the Digital Rebel XTi I'm using (blasphemy at a Nikon event, I know) feels like a fucking toy—when I fired the D3 on burst mode, I felt like Arnold firing the minigun in T2 between the rapid clacking and the weight. I've never held a more solid-feeling camera, though. (The D300 isn't exactly a featherweight either, but you won't need to swallow some spinach beforehand.) And the LCD screens: gorgeous. You could watch movies on them. Jump to keep reading and see the D300 from all angles.
I did have a bit of trouble navigating the D3's controls despite the emphatically touted Italian design and had to ask for some help, but I wasn't overly familiar with its predecessor, which could be why I had some trouble. The D300 has a simpler layout, so I had an easier time. But you can look at the huge galleries and decide for yourself on the new layout.
The one down spot is that they were really adamant about me not cribbing a sample shot from either camera to upload, the reason being the cameras are stilled considered "prototypes" and not "mass production" models. Still, it's a little disappointing/disconcerting.