At OffensiveComputing we were looking at ways to detect virtual machines and had found and discarded many unsophisticated methods such as looking for VMWare Tools running as a service or VMWare related registy keys, etc. Then we discovered Joanna Rutkowska's very interesting "Redpill" method. This was an eye opening work for us. After spending a little time playing with it we realized it wasn't fool proof on multiprocessor systems and so we decided to research the problems and possible ways to improve on the method. We discovered and implemented an improved method which is presented in the this paper.

http://www.offensivecomputing.net/papers/vm.pdf 

thanks, 

V.