There's no question that there CAN be regional real estate bubbles. But it's almost impossible on a national level (for the reasons sfrepub cited and more). Areas like the DC suburbs, much of California, etc cannot maintain the growth rate they've been seeing, but the numbers haven't been so out of whack nationally.
It's supply and demand. And unlike stocks, the demand for housing CANNOT go to "zero", it can hardly decrease at all... especially since they are "durable", but not eternal.
The real estate market is a craze right now...esp w/condos. Check out some of these tactics and tell me it doesn't sound just like the late 90s...
http://www.nytimes.com/2005/03/01/national/01spec.html?ex=1111467600&en=9a3c519c1b954155&ei=5070