Two stories look at early warnings of the housing bubble. Bruce Bartlett points to A Forbes piece from September 2001--way too early, in my opinion. Prices in most markets at that time were too low, not too high. He points to Ed Leamer's piece from June of 2002, but in that article Leamer concluded that the near-term outlook for house prices was very positive.
The Wall Street Journal profiles three long-term bears. All three of them are particularly worried now about U.S. Treasury securities being overpriced. Their views make sense to me. I am very sympathetic with Peter Schiff, who has had doubts about the dollar's ability to remain strong.