Collaborative Filtering with Temporal Dynamics: Yehuda Koren's paper (http://research.yahoo.com/files/kdd-fp074-koren.pdf) , describes recommendation algorithm incorporating time dependent info.

Koren's paper describes in part the winning algorithm for Netfix's contest, http://www.netflixprize.com//index . It is very interesting algorithm and all comments on the paper are welcome. A news article describing the team is here: http://cacm.acm.org/news/32450-award-winning-paper-reveals-key-to-netflix-prize/fulltext

A tutorial on Collaborative Filtering can be found in the second chapter of the book: Programming Collective Intelligence: Building Smart Web 2.0 Applications
by Toby Segaran. Koren did a fantastic job of adding time dependence to the algorithms described by Segaran.