I was going through my nightly reading and came across a great article at vKnowledge about application consistent quiescing. Since there are many companies using snapshot technology to take backups of their virtual machines you might want to read this article. The article goes into a good depth about the types of quiescing techniques used and what OS the technique is used on.
Taking snapshots of your virtual machines seems an adequate way of protecting your virtual machine and it application data. However, protecting transactional systems like SQL, Active Directory, Exchange and other systems using snapshots requires a little more attention.