Henry Blodget details why the Microsoft/Yahoo! deal will get more tricky over time. He describes an infinite loop of angry Microsoft shareholders who drive down the price and subsequently require Microsoft to increase share dilution as it adds more shares to get the deal done at the prearranged amount.
Blodget assumes of course that Yahoo! has a choice — either that it can not take the deal and turn itself around or find another bidder. I think either case is unlikely and they will have take whatever Redmond gives them and like it.