Can the camera's viewport be non-rectangular?

Questions & Answers
1

I want to be able to magnify objects under the mouse (like a sniper scope) with a 2nd camera viewport. Can I clip this to a circular shape?

3

Use a render texture on a circular mesh, you'll need Unity Pro though.

2

You can't shape the viewport to be a circle, but you can make a post effect (Pro only AFAIK) with a distortion/refraction circular texture that follows the mouse cursor in screen space.

4

the second camera could have its projection property set to perspective instead of orthographic