I think that if I was considering the Sony, I may as well get the MBA. I don't feel like there's any design or reliability benefit with the Sony. The Sony has better battery life. Ok. With the X300, I feel like it's a different machine, and is worth making a comparison because they are so different.
And the TZ isn't a vastly different machine?
It's a true ultraportable. It can in all manner of ways travel with you better than an Air can - starting from being able to fit edgewise into a typical 15" accomodating laptop bag, leaving room for more stuff. the Air may be thin, but that attribute - I find at least, and I have the machines we're talking about and I do travel often - is less useful in a bag-packing sense.
All-up weight for a truly useful machine on the move? Take a look for yourself (stick MP3 weight-substituted for a 3G/EVDO stick - a real one would actually be a bit heavier).
All right, the keyboard is smaller - but it's not too small to adjust to. The tiny trackpad is slightly annoying for sure. And the smaller, higher-res screen is not ideal for desktop use as a main machine due to the size. But we're not talking about a main PC here - it's an adjunct machine for travelling purposes - and the Air is outfeatured, outlasted, outweighed, outhandied, and doesn't significantly outpace the slower TZ in typical applications (e.g. With Firefox and Excel with a couple of large sheets open, Word 2007 actually opens ~4 seconds quicker on the 1.33ghz HDD TZ than Word 2008 on an SSD 1.8 Air), especially when it starts overheating. The sole area in which it is definitively better is in the pretty. But I guess a lot of people do buy based on that when they don't know what they're buying (or are blogoidiotsphere 'journalists')...
The Air has a display socket that you need a cable assembly for, a single USB socket and a headphone socket. In using it, I realise what a near-joke this actually is. Only if you are successful in deluding yourself with JRDF does it become a complete travel computer.
The TZ you see in the pictures has modem, network, Firewire, Expresscard and 2 USB sockets on the left hand side, Headphone, Mic, Memory Stick and SD card reader on the front, and a VGA display socket + DVD-RW on the right hand side. For all this there is no weight penalty. In fact it's lighter. Vista is also more stable than Leopard these days, and while I may have to forcibly power down the Air once out of ~20 times of sleeping it because it won't wake up, under Vista sleep and hibernate (you get the choice without ridiculous sudo hackery) are rock-solid. TZ + Vista is a far better travelling companion for all sorts of reasons than the Air, although it doesn't get 'ooh's from people who actually wouldn't know what a good computer was if I hit them over the head with one.