Using the middle mouse button for Zoom Pan is not so good because you very easily scroll the wheel while trying to move around causing unwanted zooming in and out. So how about using the right mouse button while holding SHIFT (or CTRL) down instead? This way the right mouse button would still retain its normal behaviour when not using the modificator key.
In general I think that using modificators (SHIFT, CTRL, ALT, and combinations) for modifying mouse button behaviour would bring a lot more fast access functionality for mouse using. Best of all if the user could specify which tools he wants on which MODIFICATOR-mousebutton combination. Everyone has their own preferences, no need to force them on the user.
regards: Seppo