will run on the MacBook Air in theory, but with the Air's average 2GB of RAM and its current choice of processor speeds, you could find that Windows may run somewhat slow on it. I'd guess that XP would be easier to run on the machine via Boot Camp than Vista, but I don't have an Air to experiment with. Then you have to factor in the availability of Windows XP, now that Microsoft has stopped selling it.
The current MacBook Air has a small, 80GB HD, and the SSD option has only 64GB. Would you really want to partition that already small space just to add on Windows? If you want to run a dual-boot configuration on the Air, and not a regular MacBook, wait for version 2. You'd probably get a bigger hard drive to work with. Course the rest of the MacBook family can carry drives with up to 250GB of storage, which is plenty for most people, and they are quite light as it is. You may want to take a look at the other Macs. It will work though, should you choose to run Windows on the Air. Just don't expect too much out of it performance-wise. Also, don't plan on making it a main computer. The Air is marketed as a lightweight portable for mobile professionals and road warriors; they usually have other computers to work on, meaning that they use the Air as a courier in a way. But if you like the Air, go for it. Just be sure to check out all of your options, especially when the regular MacBook is less expensive and more powerful than the Air.