The Microsoft Surface slates work with Miracast, which is being supported out of the box by some newer televisions. For those that don't have that support out of the box, Microsoft will apparently offer the dongle. With the FCC confidentiality agreement expiring in August, we could see the Wireless Display Adapter available before or during this summer.
What we do know at this point is that the Wireless Display Adapter is powered from the host device that it is plugged into, and it offers WiFi a/b/g/n connectivity with 2.4GHz and 5GHz support. The dongle will not work to zap the Surface screen onto another computer, even if the dongle is plugged into the HDMI port of a PC.
No word yet on pricing.
source: FCC via @davezatz, SlashGear