Yahoo May Re-Consider Google Alliance, Rebuff Microsoft
Although your idea is intriguing, I think in such cases the simplest explanation is probably the one closest to the truth. Yahoo had just announced lackluster results and their stock price was considerably lower than even a couple of months ago. It was great timing from Microsoft that allowed them to offer a significant premium to the price the stock was trading at. Anyway, as others mentioned before, a merger between Google and Yahoo would probably be blocked on anti-trust grounds.