I tested the Mouse ScrollWheel axis in my Unity (3.4, PC Intel Core Duo 1.8GHz) and it behave the same in both cases, Editor and Standalone.
The Mouse ScrollWheel axis looks more like a button than a regular axis: it returns 0.1 or -0.1 (with the default 0.1 sensitivity) for each wheel click during only one Update. In the code above, it would make the camera move 1 meter each click, what is clearly noticeable. Your script should work fine, unless some problem in your Editor is altering the input settings.
I suggest you to test this in a new scene, and if it still fail test it in a new project.