Do I think it will be the end of Halo? No. Mostly because, for some reason, Halo and Halo 2 are extremely approachable FPSs for those who aren't really hardcore gamers. Plus, the trilogy itself is just about over, and due to the momentum that the franchise has built so far, you'll probably be seeing collosal sales of Halo 3 which will overshadow by far the sales of other relatively successful games, such as GoW is predicted to be.
Other coming merchandise and games, such as Halo Wars, are a completely different genre in and of themselves, and aren't directly competing with UE3 games.
Still, the engine itself, if it proves promising, could lead to a very successful GoW franchise which could possibly aspire to the height of the Halo trilogy.
And I personally can't wait for a GoW disc to pop into my 360.