As discussed in this recent hint, 10.4.8 added a new "zoom screen via control-scrollwheel" feature. Hold down control and spin your mouse wheel up to zoom in, then reverse the process to scroll back to the default 100% scale. Someone asked me if it were possible to use a keyboard shortcut to jump back to the no-zoom view -- they found it time consuming to have to manually un-zoom after looking at whatever it was they wanted to see. I thought the answer was no, it's not possible to use the keyboard, but after some experimentation, I found a solution.
Open the Universal Access System Preferences panel, and click the Options button in the Zoom section (and make sure Zoom is enabled!). Change the Maximum Zoom slider to anything other then its default left-most position. Note that whatever you set here will not affect the operation of the control-scroll zoom feature. It will, however, affect the zoom level you see when using the Command-Option-= Universal Access zoom mode. So if you use that zoom, too, then set the slider to 20x. Otherwise, just pick any setting other than the default. Click the Done button and you're, well, done.
After zooming in, just press Command-Option-minus to return to no-zoom mode. The screen will animate out as it un-zooms, but it's still much faster than control-scrolling back out to the normal view.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20061113054029745