I see that ARRIS zooms, centered on the mouse location. That works well.
We could consider using the middle button to pan, especially if other Windows apps do it. I just tried SketchUp. They use the scroll wheel to zoom but use the middle mouse button to orbit in 3D.
The nice thing about using the same button for both - zoom and pan - is that you can focus in on a desired location without having to use your other hand, and without having to select other modes for the toolbars between zooming and panning. (In ARRIS this works well in Pan mode - you can use the scroll wheel to zoom and the left mouse to pan. However, since we do exactly this in Pan mode, it would be easy to do the same thing in "no middle button" mode - using the wheel to zoom, and the middle button only to pan)
ARRIS Development Team