One of the visitors to my site did a search for "c# googledesktop". Whilst I have never done any plugin programming for the Google Desktop, there is an article that exists called "Improving Search Precision Using Goolgle Desktop Search 1.0" this article was in the November 2005 Doctor Dobb's Journal.
It covers a C# implemenation for searching the Citeseer database with some extra information using Google Desktop. It is a nice article to read and it even mentions some of the changes required to get it working in Google Desktop 2.0.www.ddj.com
is the website, not too sure you can access the artcile unless you are a subscriber though.