Im already testing my ISDG Sceneries from XP10 into XP11. So far so good..!
So I'd say "Yes" to the vast majority of scenery out there.
CAVEAT: XP11's airport data files add more information (such as ground traffic, new ramp start types etc..) so some functionality thats "new" in XP11 -- and thus may not be 100% working when using an XP10-apt.dat until such time that the author re-issues the scenery using the new X-Plane "WED" World Editing tools.
- CK