Streaming a live battle map to the big screen with everyone in the same room is something i've always wanted to try.
You can do better than just stream your battlemap. If you launch two instances of Roll20, you can have your battlemap and stream it with only a player view on the big screen, including fog of war hidden areas and other surprises for your players. It shall become particularly interesting when dynamic lighting is going to be implemented in Roll20.
Edit: by launching two instances, I mean launching your campaign as the GM. Open another window in your browser, launch again the campaign as the GM, click the rejoin as a player button. Now you have one window displaying the GM view and one displaying the player view. You just have to stream the player view on the big screen. It works with computers. I suppose it would with tablets, but there is no way to test now.