On today's NoAH Blog (http://blogs.fp6-noah.org/noah/temporal-search-detecting-hidden-malware-timebombs-with-virtual-machines/) this is an entry on a paper out of the Computer Science department of the University of New Mexico: Temporal Search: Detecting Hidden Malware Timebombs with Virtual Machines by Jedidiah R. Crandall, Gary Wassermann, Daniela A. S. de Oliveira, Zhendong Su, S. Felix Wu, and Frederic T. Chong.

Although the paper itself wasn't available, I was able to read it via Google cache.  It certainly looks like an interesting technique.  If they can marry it to some behavioral analysis to see how it responds when a user enters a password on a web form six hours later that would be quite helpful.