The Nanum fonts, published by Naver and available for some time as free downloads, have been relicensed under the SIL Open Font License. These fonts range in size from 2.2M to 4.5M per weight (gzip compresses them slightly better than 2x), so they're bigger than what I’d consider reasonable for serving as webfonts.
For English-speaking readers, there’s lots more detail about the fonts at this typophile thread. The consensus seems to be that they’re very high quality, and also contain hints tuned for ClearType rendering in Windows.
Here’s a sample of the Nanum Myeongjo (unfortunately rendered as an image rather than served as a true font Mac OS X 10.5):
If we served these fonts as-is in the Google Font API, would there be people interested in trying them out? I’m not yet sure whether this is a good idea, so right now I’m just trying to gauge interest.
Also, if there’s anyone out there interested in working on making smaller Korean font files, please get in touch with me.