I think insulator is technically correct and I am glad we opened up the discussion on this important issue. However if someone came to me today and wanted their crawlspace insulated, I would start with insulating the floor joists and covering all plumbing with insulation. Here in the midwest, with dry winters and not so humid summers, condensation is not a problem in the summer (although it is in the winter) because the air conditioner dehumidifies the air and there doesn't seem to be a problem with the air reaching the dew point.
As a secondary measure, I would also recommend, if they want to spend the money, to put in a vapor barrier and foam insulation on both the crawl space walls and floor like you would for a basement.
I agree with insulator that it would be great to heat the crawlspace like it was a basement, I would receive skepticism from the homeowner and the local HVAC people. Our winters haven't had the severe deep cold for long periods (like canada for instance) to get people to understand this. They would just say it is wasted heating dollars.
I think that Jay and I are saying you have to be practical depending upon the region of the country you are in. I know they have different problems in Florida than we have here. Gotta go back to work. Good day.