I believe that there already is a bubble. Which makes it a rather bad risk that there will be one. :-)
What is fueling this one is low interest rates and easy loans. As a result people feel they have more buying power, and so are willing to bid up the price of property. Also a lot of people have refinanced, and while they are refinancing they get some cash and go out and spend it. (Hence fueling a big chunk of the consumer spending that has been propping up our economy.)
But where is the speculation behind a bubble coming from? It is from the other side. Banks bundle loans into bonds, and sell those to investors. Investors who are moving money out of stocks move them into those bonds. That provides lots of money to banks to lend again. Since banks actively want to make lots of loans, and have lots of money to do it with, they are in a position to offer very generous terms, completing the cycle.
So how could this fall apart?
The problem is that fairly soon interest rates hit an effective bottom and refinancings stop. At that point consumer spending is likely to slow and consumers will be in the same debt-heavy position as businesses. What this means is that everyone now needs money to make payments. If everyone wants money, and nobody wants to spend it, then you get deflation. You also get badly hurt businesses. Badly hurt businesses have to lay people off. People now cannot make their mortgage payments and want to sell their houses. But thanks to general deflation, the house prices have started to fall. So you get defaults, which dry up the supply of loans, and without those would-be buyers cannot spend as much, completing the cycle.
We just had trillions of dollars appear out of thin air and get the label of "real estate value". Just like the trillions that appeared out of thin air in the stock market, there is nothing to stop it from going back where it came from. The good times go, but the debt hangover stays with you. (And if we get serious deflation, that debt is going to really smart!)
Cheers,
Ben