The U.S. spring homebuying season has been marked by a frenzy of demand fueled by the Federal Reserve()

While values remain well below their peak, economists including Stan Humphries of Zillow (Z) and Mark Vitner of Wells Fargo (WFC) assert prices in some areas are rising at an unsustainable pace — a dramatic shift from early 2012, when billionaire Warren Buffett said housing "remains in a depression."