Some Gamespot guys are at E3 right now streaming a live feed. They sat in on an Arma 3 presentation, which should be up on Youtube soon.
There wasn't a whole lot of new information, but some in-game footage reveals or confirms the following:
In-mission weapon customization. Switch optics, suppressors, grips etc on the fly. Improved water simulation; realistic wave effects, diving, piloting of Swimmer Delivery Vehicles. Improved HALO/parachuting. Volumetric clouds which realistically cast shadow on the environment. Game world is 900 sq. km with over 50 villages, all buildings enterable and destructible. 3D mission editor Render-to-Texture (Mirrors, cameras etc) Physics - Ragdoll animations, improved vehicle collision, object interactions. Sandbox style SP campaign (no co-op campaign this time)
Video. Starts at about 1:20.
http://www.youtube.com/watch?v=V3YHbcYK82E.swf
And here is the better-quality Gamespot feed, don't know how long that will stay up. BIS section starts at 2:12 with Carrier Command, then Take On Helicopters at 2:20, and 2:41 for Arma 3;