Here's our take on the MacBook Air. It's super light, super fragile, and super small. If you just tap the screen lightly, the whole thing closes because it's so light. The keyboard looks a little weird because it's black on the aluminium, but the keyboard feels great. It feels just like a MacBook (normal) keyboard. The screen looks gorgeous—very bright and clear (and better than the Sony). It's even better looking than the MacBook's, most likely because of the LED backlit display.
The iSight webcam is a round hole instead of a square, the lid has a convex design, and slowly goes to a point on the sides. The MagSafe charger is slightly smaller, and it's silver. The MagSafe connector is also new, and connects at a 90-degree angle instead of just pulling out like the current ones. We've got photos vs. the Sony Vaio Steve mentioned during the Keynote as well, so check the gallery.
The touchpad (full impressions here) lets you pinch to expand all the pictures in iPhoto, and if you zoom in far enough you hit one picture. Once you start using the two fingers to scroll around inside one picture, it starts to lag. It's not as responsive as an iPhone. The trackpad is bigger than on current laptops—not so narrow but more fat. The button (mouse button) is smaller than a MacBook's though.
On the top of the screen, next to the iSight, is an ambient light sensor and a mic. Since the bottom is curved down, it doesn't have a hard 90 degree edge that you hit your hand on when you're typing. Very nice.
Verdict? Ultra light, ultra awesome.