A couple of really simple things explain this sale. There is nothing complicated about it and it certainly doesn't reflect any diminished interest in Windows Phone. It is freakin' amazing how quickly some folks leap to the worst.
First, it is between that time of year when retailers go all out to draw people into their stores with deals. Second, the economy still pretty much sucks and retailers are amping up their usual promotions and deals more than ever before in hopes of an improvement over the lackluster Christmas season of 2009. That pretty much explains what is going on. Through in the ads that Microsoft is running for WP, and the carriers can leverage that publicity to get customers in to their stores.