Looks like it'll be an exciting shareholder meeting... In the meantime, Yahoo!'s stock is up ~12.5% so far today. The official Microsoft response to Carl Icahn's recent message can be found on this page.
Yahoo responded that it continues to stand ready to enter into takeover talks with Microsoft, and feels "very strongly" that a deal being reached after the installment of a new board "would not lead to an outcome that would be in the best interests of Yahoo's stockholders."