Is it possible to view the scene in the viewport camera view at 1:1? I'm manipulating some things in the scene from camera view, and I'd like to be able to tell exactly how big they will appear in the render.

Is there some way to make the camera view zoom to 1:1 in the viewport, without rendering the scene?
Yes, Blender has this feature:

From the 3D View Header:

View -> Navigation -> Zoom Camera 1:1 (Shift + NumpadEnter)