Not a flame, but...
Not wanting to flame, but the difference is not small, DEPENDING what you want to use it for...
1.6GHz Atom, which is the best you get in Netbooks yet, is a single core CPU that is roughly on a par with a 900MHz Celeron. It could probably be outperformed by a late model G4 at a guess, though don't quote me on that.
Netbook> max 1GB RAM with the majority of chipsets
Netbook> small keyboard, small screen, lower resolution
The *REV B* Air is, for me at least (though not everyone) capable of being a primary computer. Its keyboard is vastly superior to any netbook for extended typing unless you have child-size fingers. Its screen is higher resolution than any netbook. The closest would be the HP Mininote which has a 1280x768 model which is close to the Air's 1280x800 but several inches smaller.
The speed difference is not trivial - it is vast. I don't have benchmarks, but the Air Rev B has a proper 45nm Core 2 Duo 1.6Ghz or 1.86GHz. Turn off one of the cores and it would still wipe the floor with a netbook's CPU. In short its processing power is probably several times that of, say, an MSI Wind. As for its graphics, there's no contest whatsoever.
I'm not saying there's anything wrong with netbooks, if they're what you want, but they're quite quite distinct categories of machines.
The wonder of the air, for me, is in being as light as it is, yet being just as useful (to me, since I don't need optical drive and its fast enough etc etc) as a macbook or macbook pro. The closest competitors are things like the Voodoo Envy or the Lenovo X300 which often work out more expensive than the Air, depending on model. And in the Lenovo's case it's still much more powerful than a netbook but not on a par with the Air. And of course they run Windows.
Granted the Lenovo and the Voodoo do better on the ports front, but I want a Mac, and most of their ports I don't need. The lenovo also has a smaller screen, and I find 13.3" about as small as I'm comfortable using for my main screen.
Plus, find a PC laptop that can drive a 2560x1600 external display and I will show you a complete beast that weighs 3kg plus most likely. Even in that weight range it's rare - some Sonys and a few Acers and that's about it. Certainly won't be 1.36kg. I intend to get a 30" display when I'm not spending all my money on repairing dents
Few netbooks will drive a 1920x1200 24" display, let alone 30"
For all that, I will put up with the single USB port (though another would be nice). 4GB RAM would have been nice though, but I expect Airs will be available with that before long. And really, 2GB is enough for my needs - I'd just gotten used to 4GB in a MacBook and I've not really noticed much difference since I don't do image-heavy stuff.