i dont see why not. But i think it will just notify you that Lion is available for an upgrade, and it will redirect you to the app store for the download. That way Apple can get your $29.99.
Well, probably. With iOS, there have occasionally been differences between the GM build and the release build. (For instance, presence of Game Center in iOS 4 release.) It's not going to be certain they're the same until we see the build that comes out tomorrow.