Wouldn't the declining value of Yahoo over time have something to do with it? Yahoo stock was as high as $30 in early 2007 and was trading at under $20 when Microsoft made the announcement. Certainly the stock price has bumped up but imagine what Microsoft would have to put up if Yahoo was trading at $30 before this potential deal was announced.
Also, we have yet to see what price the Yahoo board would actually sell at. In light of their saggy long-term growth outlook and numerous attempts to run away from the deal (deals or mergers with Google or AOL) a decent stock bump from a News Corp/Microsoft combination would test their sincerity re: a higher price.