Actually, there was a great article in the Economist on this. Some of their predictions seemed a little dramatic, but many of the reasons they citied that they believe that this is a bubble seemed legitimate. I know that people have talked about this for a couple of years, but I think the talk is getting more serious. We're also starting to see the market cool in some places overseas and even domestically (I mentioned earlier that Orange County and San Diego County had both seen some cooling). Most of the people that are taking the "There's no housing bubble!" stance seem to be in real estate or home building.
I think all the talk of a housing bubble will cool the market a little (investors may back off).
We're starting to see more economists talking about seeing a bubble. There were several reports release just last week -- UCLA was one of them.
Without the help of "experts" though, I think some housing markets have to be at their peak. There are lower-middle class neighborhoods near me that have houses selling for $500-600k. That's nuts. Who's buying these houses? Houses in better areas (i.e. better school districts) have prices at about $700-800k. Someone I know lives on a street with, in my opinion, fairly modest houses selling for over a million dollars. These houses would have been $600k, at most, 5 years ago.