The Economist has an article about the global housing boom which has some interesting historical data and seeks to dispel the housing myth which states that "what goes up keeps going up".
They rely on historical data from around the world and point out that the ratio between housing prices and rent has risen and is substantially above historical levels and also that housing prices as compared to incomes have risen as well. The Economist does a good job of supplying data to back their assertions.
This has certainly become a more carefully watched trend over the past couple of months, but to me the most worrying trend is that towards riskier loans. The Economist states:
Interest-only mortgages are all the rage, along with so-called “negative amortisation loans” (the buyer pays less than the interest due and the unpaid principal and interest is added on to the loan). After an initial period, payments surge as principal repayment kicks in. In California, over 60% of all new mortgages this year are interest-only or negative-amortisation, up from 8% in 2002. The national figure is one-third. The new loans are essentially a gamble that prices will continue to rise rapidly, allowing the borrower to sell the home at a profit or refinance before any principal has to be repaid. Such loans are usually adjustable-rate mortgages (ARMs), which leave the borrower additionally exposed to higher interest rates. This year, ARMs have risen to 50% of all mortgages in those states with the biggest price rises.
This adds an extra level of volatility to the market - especially where I live as I've heard that locally interest-only loans account for 75% of new loans. A negative change in market value or interest rate can quickly put those people underwater and force sales, leading to a downward spiral.