answer: there is none. It's only a matter of time before Google consigns them both to the dustbin of tech history. And in case anyone's interested, I'm long MSFT & YHOO...but don't ask why? :(
Remember that we don't allow monopolies in this country. Google could end up going the way of US Steel or AT&T or even Microsoft for that matter.
My take is that longs are being very cautions at this time. They do not want to raise the bar this time but they know this partnership is very positive to YHOO. Myself, I will take small gain to huge gain (means buyout later).