It is: just the Control key + scroll mouse wheel up, and the entire screen will zoom in. Scrolling the wheel down will zoom out. On Mac notebooks one can hold the Control key down and scroll trackpad with 2 fingers to “scroll.” (I believe the “jaggies” will be fixed in the much anticipated Leopard.)

Here is another method for zooming whole screen:
Option-Command-8=Turn on Zoom
Option-Command-+ (plus)=Zoom in
Option-Command– (minus)=Zoom out

Using the keys allows one to zoom to the most comfortable level and leave it that way with out having to scroll…

To enlarge only font size in some apps, most notably browsers, one can Ctrl-Opt-Command, scroll.

And Opt-Command (at least in FireFox) will flip through previous web sites visited. A fast “back” or “forward” if you will. Not tested in other apps.