Mac OS X Netbook Compatibility Chart (Updated June 2009: Sony Vaio P and MSI Wind U120)
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.
The short answer: get an MSI Wind, Lenovo S10 or HP Mini 1000. Gizmodo has an excellent how-to for the Mini 9, but it's no longer being sold.
(Updated: June 19) MSI Wind U120, courtesy of Matt Hickey.
(Updated: June 13) Vaio P.
(Updated: May 22) Added a bunch of new machines.
(Updated: Feb 1) More columns! Lenovo's S10 bumped to recommended machines: prettier than the Wind and has a 10" display, so unless you need Ethernet, consider it.
* Aspire One WiFi only available by replacing adapter
* Frequent kernel panics reported during Samsung NC10 installs.
* Dell Mini 9 requires SSD replacement for sleep.
* Fan control and ExpressCard34 work on Lenovo S10.
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.
Post updates and corrections to the comments (or to me at rob at boing boing).
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.