Update: This has been useful for many people, which is very rewarding, but I don't have time to maintain it anymore. Sorry! Instead, a straightforward answer: Get a Dell Mini 10v and use this guide to put Snow Leopard on it.
Apple says it's had "pretty interesting ideas" for a Mac netbook, but it's everyday users who've already taken action. Most of the popular netbooks can run OS X, but there are plenty of caveats: non-functioning components which lack drivers, need to be replaced, or which simply can't be gotten working at all.
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Frequent kernel panics reported during Samsung NC10 installs.
Fan control and ExpressCard34 work on Lenovo S10.
Email us or add a link in the comments if you've got OSX working on a new netbook. We'll add it to the chart and link it to your instructions. Thanks!
Note: In many cases, components can be replaced with third-party parts that work with OSX, especially WiFi. And you can always just use USB dongles.
Note: Green doesn't mean it's easy to set up, or that you won't need third-party software. It just means you can get it working. How-to guides for each netbook are linked to from the chart.
Hacking OSX into a netbook isn't easy, and may require familiarity with the terminal. You'll have to grab a hacked version of OSX from the tubes, in contravention of Apple's EULA. You should own a legal copy, too. Piracy isn't nice.