mpicpp writes with news about a new Microsoft trade-in program to encourage sales of the new Surface Pro 3. Microsoft is offering a limited time Surface Pro 3 promotion via which users can get up to $650 in store credit for trading in certain Apple MacBook Air models. The new promotion, running June 20 to July 31, 2014 -- "or while supplies last" -- requires users to bring MacBook Airs into select Microsoft retail stores in the U.S., Puerto Rico and Canada. (The trade-in isn't valid online.)...To get the maximum ($650) value, users have to apply the store credit toward the purchase of a Surface Pro 3, the most recent model of the company's Intel-based Surface tablets.