Exactly!
It depends on what you are doing. For reading documents, mail and surfing, the 12" screen is fine. I have also edited photos and videos on long trips and it is OK. But for software development or photo/video work I prefer a large screen.
As for the keyboard, the sizes are identical on the 12" PB and 15" (17"?) PB.
The point is that the laptop is not limited to the built-in screen. And if you are spending a full working day at a desk you better get a large decent screen and external keyboard/mouse. The ergonomics is so much better. Even if you've got a 15" MBP. Still I see people buying large laptops and use the built-in screen on their desks for whole days. Ludicrous!
Of course, if you do most work at various hotel rooms, you need to use the built-in screen and may want a larger one.
I would be fine with a 12" screen too, provided it was widescreen and at least 1280 x 800. That reminded me that one thing that I have gotten "spoiled" to in my MBP is the 1440 x 900 screen. I don't really give a rat's behind how big the screen is (within some reason), as long as it's a usable dot pitch (like the iPhone's dot pitch, 160ppi or whatever it is--no denser than that), and a biggish resolution. 1024x768, the resolution of the old 12" Powerbook, was
gut-wrenchingly bad, in my opinion. You couldn't fit near enough on the screen. I can't tell you how, but 1280 x 800 is significantly better, even though the real estate increase is not exactly dramatic.
I wish Apple would apply the "high-res" edition that they have for the MBP17" (the 1920 x 1200 screen version) to the rest of their line. As in, you pay some extra, and you get a 1600 x 1050 screen in the MBP15", or a 1440 x 900 screen in the MB/MBA.
Like you say, it's also possible to fit a full-size keyboard into a 12" laptop, but there would be NO bezel (like the PB12") if you had a standard 4:3 aspect ratio, and there would be only a small one if it were widescreen. The problem, though, with a 12" widescreen is that you are going to end up being stuck with a shrunken trackpad, and I would guess that a trackpad smaller than the one that was on the PB12" would be a real pain to use. I am looking forward to trying out the big trackpad on the MBA.
I am not totally sure that you could get a full keyboard and a good-sized trackpad into a 12" widescreen laptop format. Probably it's possible, but I don't know. I think if they did it, it would indeed be gorgeous, and if it had a high enough resolution I would be gushing over it as well.
I just like the MBA because I am an optimist. There are things that I wish it had, and criticisms I can think of, but it's just so beautiful and cool to me, and, of course, the crux of this, it's
enough for me (or rather, it will be with rev. 2--considering my optimism).