The advertising alliance between Google and Yahoo! has collapsed after the Justice Department indicated its intention to try to block the deal.    Earlier this year the Yahoo! board rejected a takeover offer of $33 a share from Microsoft, in a deal worth $47.5 billion, claiming it undervalued the company and that Yahoo! shareholders would be better served by the company following its own strategy.   Part of that strategy was to hook up with Google in an arrangement where paid search ads from Google would appear alongside Yahoo!