I was thinking the same thing a while back. An API for HN would be awesome. A problem with that though is that it is really only a one way street and only allows for passive consumption rather than easily interacting. While a site-scraper API implementation would be handy, I think people would lose out on the functionality and community interactions that can only really be had at news.ycombinator.com.

Still though, I suppose some sort of site-scraped data (perhaps hosted on google apps for everyone to use) could be used in tandem with a greasemonkey script to further enhance the site without taking away the interactivity, that could be interesting...