A filing with the FCC reveals Microsoft has been working on an accessory currently called the Surface Wireless Display Adapter. The accessory is an HDMI dongle which allows mirroring to a tv or other external monitor from a Surface 2 or Surface Pro 2. According to the FCC filing the dongle supports dual-band WiFi 802.11 a/b/g/n and may hit the market as early as August. It is possible a demo may be shown at the Build Conference in a few weeks.
It should be noted the Surface 2 currently supports the Miracast standard which allows you to mirror your screen to another Miracast capable display wirelessly.