Can’t fault them for trying.
Similar to their iPad trade in, if you’re carrying an iPhone, but you’re really curious to see how the other side lives, Microsoft is offering a minimum trade in of $200 for your “gently used” iPhone 4S or newer handset. Those folks I saw at Social Media Week rocking smashed iPhones need not apply (but seriously, how do people use phones like that without slicing their thumbs open).
While this promotional offer is  surely convenient, you’re apt to do better on the open market. The iPhone 4S is currently selling for around $250 on ebay, and if you throw in those accessories you don’t need anymore, like cases and old dock connector cables, some are even hitting $300.
Still, if this is your jam, the offer is good until November 3rd.