It looks like MSN Desktop Search has (or soon will have) an API.

http://addins.msn.com/devguide.aspx#WindowsDesktopSearchApi


I was aware of the iFilters API but not the other stuff. It does set the mind a whirring. I find the API to add you own protocols for accessing custom stores most intriguing.

[Edit: Oops meant that to go in Coffeehouse]