I just read this interesting tidbit on Ask Yahoo.

... According to a 2001 study by the Census Bureau and the Department of Housing and Urban Development (HUD), "nearly 40 percent of all residential properties in the United States, owner-occupied and rental units, are not mortgaged but are owned free and clear." For a country so often criticized for its debt, that's not a bad figure.

Despite that, this is also true:

Those who wish to learn more about the demographics of the average American homeowner (be it person or corporate entity) can skim the report's 368 pages of scintillating facts and figures. One factoid that stood out to us -- from 1991 to 2001, the amount of outstanding mortgage debt on single-unit properties rose from $1.62 trillion to $3.48 trillion.

It's really amazing when you figure that the $3.48 trillion is only financing 60% of the homes.