"After a series of meetings over the past week, Yahoo's board determined that the $31 per share offer "massively undervalues" Yahoo, the person said...The board plans to send a letter to Microsoft on Monday, spelling out its position."So let us wait & see how it unfolds.
UPDATE: Here's the official confirmation of this news from Yahoo.
Yahoo's Press Release : Yahoo! Board of Directors Says Microsoft's Proposal Substantially Undervalues Yahoo!