Robert Mullen has raised almost $1 billion to buy distressed single-family houses across the U.S. to rent. He intends to join other financial giants in taking advantage of a 29% dip in home prices since the 2006 peak and more demand for rentals for Americans who are not yet able to buy homes. Fundamental REO, his private equity fund, has already boughtalmost 2,500 homes through government agencies, auctions and even an affordable housing non-profit in Arizona.