ICAO address is used as unique airframe identifier for so called Mode-S transponders - http://en.wikipedia.org/wiki/Aviation_transponder_interrogation_modes#ICAO_24-bit_address.
It is likely at least in Europe that beyond line of sight UAV operations in future will require equipping UAVs with such transponder.
So we decided to start working on ADS-B OUT transponders integration with popular autopilots and adding these features to our ground control software UgCS (www.ugcs.com).
As first carrier we have selected 3DR Quad with Pixhawk.
Why Pixhawk? - This line of autopilots have matured and we expect lot of commercial applications will be using it. We will be ready for that with our solution for Pixhawk- ADS-B transponder integration.
We selected 3D Robotics Quad because we have lot of them and they are symbolic for getting quads and UAV technology to the masses. :-)
We will use Sagetech XPS-TR transponder because of its small size and weight, which allows equipping even lightest UAVs with it.
4 months ago we approached Latvian Civil Aviation Agency with request to provide us ICAO 24-bit address for our quad, and now it has happened!
Latvian Civil Aviation Agency (FAA analogue for us) has issued ICAO address 502С08 for our little bird - 3DR Quad with "tail number" 6 (see picture).
In a month or two it should appear on flightradar24.com.