Wow, I just read this on the Wall Street Journal.
This strategy was touched on in the House of Cards documentary.
Having done some reading lately, it appears that there were several obvious signs, this article references a few.
It seems a few people got it, the fundamentals which other seem to have ignored having being blinded by their own succes.
John Paulsons hedge fund was up around US$15 billion in 2007 having bet against the US housing market.
http://online.wsj.com/public/article/SB120036645057290423.html