I'm a believer in this concept. I don't normally use a virtual machine for browsing, but I do when I'm accessing the web for financial purposes (viewing bank statements, credit card purchases, etc.). By resetting the VHD file to a template after every use I'm assured that no virus can gain a foothold. It's not foolproof (ultimately there still has to be a "real" host system at the top of the virtualization stack which is still vulnerable to things like keystroke logging or network packet interception), but it's another layer that makes me a harder target for things like identity theft.