Here's the article that I based most of my comments on. It rings true to me.
http://www.buildingscience.com/docum...ace_2010r2.pdf
My understanding, such as it is, is that the moisture condensation problem common to walls and floors are very severe with floors with vented crawl spaces that have insulated but partially exposed floor joists. It's actually a lot worse than walls because warm air travels up and also there is little sun or air circulation to dry it out that walls might have.