Pesky competition laws
The Wall Street Journal had dug up its usual "people familiar with the matter" to suggest that both parties may be about to walk away from its proposed advertising partnership between Google and supposed rival Yahoo!
You have to subscribe to see the whole WSJ piece, but Reuters has written a piece regurgitating the WSJ rumour. It adds that the two companies collectively owned more than 80 percent of the web-search market in August. If their combined efforts don't constitute a monopoly then it can't be far off.
Yahoo! rejected a $33 per share offer to be bought by Microsoft last June. Soon after the rejection it announced the Google deal, presumably as evidence of how it was going to raise its share price beyond $33 without Microsoft's intervention. At close of markets yesterday Yahoo!'s share price was $12.93.