This is a nice article on the matter: http://web.archive.org/web/20110929075510/http://invisibleth.... It doesn't appear to work reliably anymore, but the principle is the same.

Even if we reached the point where VMs never technically revealed they were VMs, I think the human element might still play a part. When I spin up a VM for a quick test, I might leave it on 512MB or 1GB RAM - but this could be a giveaway that it's a VM (not 100% of the time, granted) given how unlikely a fresh installation of Windows is to have that small amount of memory.