According top a recent blogpost, Yahoo is not the kind of company to just give up the fight against a rival who has yet to score a major victory. Instead, the big Y would be much more inclined to ask Google for help:
“Yahoo! is famously independent and as an organization, has held looked at Microsoft with disdain. We believe that they will do *everything* possible to avoid being acquired. Their most likely savior would be Google, in that Yahoo! would turn over all of its search advertising to Google in exchange for a guarantee. If Google was willing to pay $900 million for MySpace’s search advertising, Yahoo! would get a huge premium. That would be enough cash to bolster Yahoo!’s efforts to reposition its efforts as a portal and display advertising powerhouse.”
On the other hand, even if the acquisition went through, Microsoft would face major merger-related problems:
“acquisitions are messy, ranging from trying to consolidate advertisers on one platform to figuring out what to do about competing brands like Yahoo!, MSN.com, and Windows Live. Cultural differences, geographic distances, and different technology platforms will also muck up things, […]And of course, Google won’t be standing still and instead, will capitalize on advertiser and user confusion.”