Tysons IS an indoor mall, yes. I don't know if they are actually open all night, I doubt that, but when I went for the 3G iPhone launch, I got there at 5:30 or 5:45, something like that, and was able to enter. I believe that they had only the one entrance open, though, the one which is closest to the Apple store, and mall security was around there, so I'm guessing that they did this specifically for the Apple store and the launch. I would expect that the situation will be similar on Saturday.
I was at the Apple store last night and one of the employees mentioned that he'll be there at 7:00 AM that day, presumably to assist with getting things set up.
No idea about how they'll do things at Fair Oaks. It's a smaller Apple store than the one at Tysons, if I recall correctly. I've only been there once and don't remember where it's located within the mall.