I'm not sure if this is new to version 10.5.6 of Leopard or if it's always been this way, but there seem to be setting that allow you to Control using the mouse to zoom in on the screen. You've always been able to zoom in by holding down the Control key and then scrolling the scroll wheel on the mouse. But now you can change the key that controls this as well as the movement of the zoomed image.

To get to this go to System Settings and click on the Trackpad option. You can then change the key used to control scrolling from the default of Control to either the Option or Command key as seen here:



If you then hit the Options... button, you can control how the zoomed in image moves around. The default is that when you move the mouse, the entire screen moves. But now there are options so that it only moves the screen when the mouse hits the edge of the visible image, or an option to keep the pointer always in the center of the image so that it's easier to find.

Like I said, not sure if this is there on previous versions of Leopard or Tiger or not and I never noticed them or if these are new.