I would like to see a side by side photo of the ipad and an Air or 13 Macbook. Is it really so significantly smaller that it actually means something? Its not like it would fit in your pocket like an ipod or iphone. If you need to carry something that size, why not carry a laptop?
Really, the 3 pound difference in a piece of carry-on means nothing when i can carry on a 35 pound Phenom with camera PLUS a backpack..
They don't have photo side-by-side comparisons to show scale, that I see, but:

Height: 9.56 inches (242.8 mm)
Width: 7.47 inches (189.7 mm)
Weight: 1.5 pounds (0.68 kg) Wi-Fi model;
1.6 pounds (0.73 kg) Wi-Fi + 3G model

Width: 12.8 inches (32.5 cm)
Depth: 8.94 inches (22.7 cm)
Weight: 3.0 pounds (1.36 kg)
Perhaps once the things are actually shipping people will take some photos of them stacked so we can see better.
Of course, you can buy two iPads (or 3, depending on configuration) for the cost of one MacBook Air.
And personally, I can't stand the limitations of the MacBook Air (from a connectivity standpoint, among other reasons). If I want a laptop computer, the 13" MacBook Pro is as small as I can see going, because that's the size you still have reasonable connectivity options, IMO. Unless funds were unlimited and I could have one of everything, of course. It's a lot easier to justify adding an iPad to my existing collection (iPhone and several laptops) than the MacBook Air.
And again, can you get 3G internet connections with the MBA? Also, what about battery life? What's the battery life on the MBA, 5 hours, best case, vs. 10 hours of VIDEO playback on the iPad?