Redmond has not mentioned the dongle so far but it did pop up in an FCC filing.
The filing lacks much info to identify the device, but it carries the model number HD-10.
The FCC filing says that this device has an HDMI port, Wi-Fi and a USB charging unit.
But if you look at the Wi-Fi Alliance product database you can spot that Microsoft’s HD-10 is described as a Miracast dongle.
Miracast is a wireless standard that lets devices connect to one another and share media. It is not as sexy as the Chromecast or Apple AirPlay. Miracast doesn’t let users queue up multiple files from different sources or play multiplayer games, and it requires media to be played on other devices and sent to the TV, rather than directly from online and cloud sources.
However, this does mean that Microsoft will get its Windows Phone 8 and Windows 8 devices casting to the TV which, at the moment, it cannot do.