I wonder if this will be like the 360 BC with them running a virtual machine that runs the games or if it'll be the emulator used on the 360.. as in, they'll use the 360 backwards compatibility to run the emulator they made to run the original xbox games on 360.. I'd assume the latter would be less work as it'd just be like running any other 360 games on the Xbox One but I'd think the games would run better if the did it from scratch using a custom VM or emulator made for Xbox One.
If they go down the route of using the emulator from the 360 days, I guess we'd get the same games that worked on that (baring any games that now have licensing issues) but stuff like Black, Ninja Gaiden, Fable, Evil Dead games, Buffy, KOTOR 1 & 2, Jade Empire and Burnout 3 would be nice - not to mention of course, Shenmue 2, Gun Valkyrie, Jet Set Radio Future, Panzer Dragoon Orta.. so much Sega goodness to rediscover