Biz: Why MS walked out on Yahoo?

Probable reasons why Ballmer walked out on Yang & Co.:

• A prolonged proxy battle makes both MS and Yahoo take eyes of the ball
• Yahoo and Google have done well in the initial ad. tests making cooperation all the more possible, hence taking the price a too high to be comfortable
• Anyways, MS is making huge strides in the display ad market
• And finally, the anti-trust issues on mail and instant-messaging will make takeover proxy look easy

So, I think Ballmer chose to take the slower but more appropriate path of facing Google on its own effort and not with external support which will be painful, treacherous and hostile to say the least!