At the Beyond Personalization 2005 workshop early last year, several people from Yahoo presented a short paper, "Recommender Systems Research at Yahoo! Research Labs" (PDF).
The paper describes "some of the ongoing projects at Yahoo! Research Labs that involve recommender systems ... and solutions relevant to Yahoo's business."
The paper is too short to have much detail, but it does give some examples, including a content-based movie recommendation prototype that sounds vaguely similar to some stuff at IMDb.com and a more unusual project attempting to make music recommendations based on similarities in the raw audio streams. It also briefly discusses some more general questions, including the cold start problem and using content versus user behavior data.
The paper says that Yahoo plans to integrate recommendation technology into Yahoo Search and Overture, among other places. Personalized search and personalized advertising, coming to you soon from Yahoo.