If you are using XL2007, the F6 key toggles between the Zoom Control (in the
lower right corner of the window), the display of the Panel's shortcut keys
and the worksheet grid. When the Zoom Control is highlighted, you can use
the right/left arrows to highlight the + (plus) and - (minus) icons, as
needed, and press the Enter key to zoom in or out (just keep hitting the
Enter key as many times as desired).