Apple iPhone 3GS, iPhone 4, and iPod touch 3G support Zoom In and out features for accessibility reasons, this functionality is meant for users having difficulty in viewing small text/graphics. If you have left this feature ON and accidentally triggered the Zoom (double-tap with three fingers) leaving you stuck with a Zoomed-in screen here are the steps to get things back to normal by resetting it to the default zoom values.

First of all to bring back Zoom level to normal use three fingers to double-tap and drag adjusting the magnification to normal, now to turnoff the feature completely navigate to Settings -> General -> Accessibility -> Zoom -> Switch Off.