By the end of this year as many as 15 million U.S. households may owe more on their mortgages than their homes are worth, according to an estimate from Jan Hatzius, chief U.S. economist of New York-based Goldman Sachs Group Inc. That may fuel an increase in foreclosures, erode prices, and increase mortgage bond losses, he said in a Feb. 1 report.
...Thirty-nine percent of people who purchased a home two years ago already owe more than they can sell it for, according to a Feb. 12 report from Zillow.com, a real estate data service. Only 3.2 percent who bought five years ago are in that situation, the report said.
This excerpt is from Bloomberg does not separate the two different reasons people are under - so bought at the high and put little or no money down. Any drop in price makes an upside down mortgage. Some people did this on purpose when buying recently using an option mortgage and generating negative amortization. The other group bought when the market was lower and than HELOCed the hell out of their property.