Qualcomm today announced that it has found a solution to allow wireless charging for devices with metal exteriors. It uses Qualcomm WiPower technology and the techniques for designing a device to charge through a metal back cover, as well as the full suite of WiPower reference designs, are available today to WiPower licensees. I think Microsoft is not a licensee of WiPower yet, but they are a close partner with Qualcomm which will allow them to use this tech in their future Lumia devices.
WiPower, as well as other technologies that meet the Rezence standard, operate at a frequency that is more tolerant of metal objects that come within the charge field. Up to now, this meant one could typically have objects such as keys and coins in the charge field and not affect the charging process. Today, WiPower has added the ability to have the device itself made of metal. This advancement maintains WiPower’s existing ability to charge devices requiring up to 22 watts today, at speeds equal to or faster when compared to other wireless charging technologies.
Read more about it here.