Yeah, so this came out of the blue at E3 2015 as well.
I was quite annoyed that XBLA games were not able to be transferred over to XBONE in the past, but obviously, reasons.
I've got a large library as I am sure most of us have here. So it's a good thing that I could go back and revisit some of these games and polish off a few that I never got time to doing previously. What was surprising was the fact that disc based games will work as well.
At the very least, this might put a halt on all of these "Remasters" that have been filling up the XBONE library at the moment.
I've got Geo Wars 2, Hexic, Perfect Dark and Battle Block Theater on my Xbone working at the moment. Everything seems to be smooth enough so far. It's only available to Preview Members at the moment, but if anyone wants an invite, let me know and I will send you one.