Microsoft has proposed to acquire struggling Web portal Yahoo! for $44.6 billion, or $31 per share -- a significant premium over yesterday's closing price. The offer was contained in a letter sent Thursday to the Yahoo! board by Microsoft CEO Steve Ballmer. In an accompanying statement, Ballmer wrote, "We believe our combination will deliver superior value to our respective shareholders and better choice and innovation to our customers and industry partners." Yahoo! issued a statement saying it intends to "evaluate this proposal carefully and promptly in the context of Yahoo's strategic plans and pursue the best course of action to maximize long-term value for shareholders."