A number of people have given various objections to the Macbook Air (MBA): small drive, no firewire, no ethernet port, and so on. I can live with all of those. I'd get one primarily for travel, so I don't mind the small drive. I've been using disks over 801.11N with my MBP for months and won't miss an ethernet or firewire port. Further, I'm intrigued by the solid state drive.
So, the MBA looks like the perfect travel machine with one exception: there's no 3G card. Huh!?! Further, because there's no Express/34 slot, I can't use the card I already have. I'd have to go get one of those USB soap-on-a-rope aircards. Ugh!
Now, if I could tether to my iPhone and use the network from there--I'd be OK. But, alas, you can't do that without jailbreaking the iPhone. So, for now, I'm passing on the MBA.