I've tried looking this up, but couldn't find anything, do we know? Or is it simply to big of an accomplishment, when we're talking about all of tamriel? Please let me know, because it's a pretty big deal for me since i'd like to be able to interact with my enviroment.

I would guess that Vendors, Quest Givers and some explorable houses are open. But I would not expect every single building to be enterable.