Properties support for RandR 1.3 has been discussed for quite some time and was discussed again in October with an initial proposal. This RandR 1.3 properties support has now been committed to master in the respective Git trees and the RadeonHD driver has picked up support for the required properties.
The known properties for RandR 1.3 include EDID data, signal format, signal properties, connector type, connector number, compatibility list, and clone list, but only the signal format and connector type are mandatory to be a RandR 1.3 compliant driver.
The latest RandR protocol specification can be read about here. Additional information about this new functionality that's exposed to RandR clients can be read about on Matthias Hopf's blog (the one responsible for much of the RandR 1.3 work).