First, I've since learned that this position has been open for two years and they have hired no one. They love to interview people but never want to hire anyone. 3 recruiters have independently told me this exact thing.
So...
Two part interview consisting of a written assesment and a face-to-face with 2 devs and a manager.
The written assessment was 5 pages, 3 were C#, last 2 were SQL. On C#: know abstract classes, interfaces, and inheritance (and how they all intermingle). Know delegates and reflection. Know about multiple interfaces and how they interact when the function signature is the same. For SQL, know basic triggers, table relationships (1-to-1 vs 1-to-many vs many-to-many). Know what "NULL" really means. (google it.)