I am a hardcore traveler (travel on a week on/week off schedule), while I have not used a Macbook Air, when I am on the road I do wish I had it vs a Macbook.
The macbook is smaller than most laptops, but it weighs the same as other laptops. After you add all the other stuff in your travel bag (work equipment, a personal book, magazines, ipod, etc.) and the standard travelpro roll-on luggage the weight really starts to add up.
The downside is that I don't believe that an Air could be your only computer, while i am able to use my macbook (with upgraded hard drive) without issues, just unplug and go. The Macbook allows 4 gigs of ram and uses a standard 2.5 hard drive, vs the Air's fixed hard drive and ram.
The Air's price is also a big issue, you get a "real computer" vs a netbook, but you do pay for it (unlike a netbook.)
At some point i wish apple would release a netbook (I know the rumors exist that they are working on one, but who knows if it will ever become a reality.) I have decided to try the Dell Mini 9 and do the unofficial Leopard
install, but that is a really ugly solution vs a real mac netbook.
(I would never consider a hackintosh as your only Macintosh, but just as something to toy with, one update from apple and they could break you computer.)
Good luck with your buying decision.