I wouldn't call the TZ a marvel.... I have one, cramped key board very small screen, very slow operation (XP or Vista)...it is a nice sub notebook, and has served me well, but trying to open two windows on the thing and typing more than a paragraph is painful. I just got my air and in my opinion the build quality is much better the keyboard is heaven and i love the screen. And its running OS X and is even pretty snappy with Vista in VMFusion. Give the decision to offer a 13 inch screen and full sized keyboard seems to have been paramount i think the engineering Apple put into it is impressive, and I think its looks soooo good.
Couldn't agree with you more on TZ and other subcompact. They all focus on what they can pack in as small of space as they can, so they miniaturize even the ergonomics; screen size, keyboard and CPU power. After years of using subcompact, I have to say MBA have all the right criteria for most people whom need portable computing. I used to think I could not do without a DVD burner, but in reality I have maybe used it to reinstall system (Windows).
I used to think I need at least 3 USB ports, but after years of use, I really only need 1 port for iPod / iPhone sync and external drive.
The reality is that if most people pay attention to their daily use, they will find that all the 'missing' ports and interfaces are not necessary, especially trading for CPU horse power like the TZ and Toshiba R500.
Yeah the TZ have higher resolution screen, but if it requires a magnifying glass to read, it's not usable on the plane or work with on the road for long period of time.
So IMO, MBA does hit the mark with most user of their daily computing needs..
Just my 2c