There were articles a week ago about them raising $150 million in a new round of funding, with a valuation of $2-3 billion. So this isn't out of nowhere. Also, they raised $80 million a year ago, at a valuation of $800 million. (Incidentally, Google Ventures led that funding round, so at least some of this purchase price is just Google moving money around internally.)

http://gigaom.com/2014/01/02/why-nest-might-need-another-hug...