Yes I beleive equity is EXTREMELY skewed especially across different regions. Further if that number contains corporate re and loans that would be true even more so.
I think when you adjust for credit debt, yes the majority are underwater and further they expect their 401 will bail them out. Which is might, they may end up above water. But not where they wanted to be.
A downturn in stocks or real estate would severly affect the average homeowner.