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.
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.