At 62% above their current share prices, it may just be an offer that Yahoo can't pass up.  The combination of MS + Yahoo may also actually form the first viable competitor to the massive Google conglomerate, which has dominated the online application & search world as of late.  Either way, ~$45 billion is a whole lotta moolah...

 "Under terms of the proposed deal, Yahoo shareholders could choose to receive cash or Microsoft common shares, with the total purchase consisting of 50 percent each cash and stock."