As many have discovered, the trick for scrolling in list boxes on the iPhone (using two fingers) doesn't work in form boxes. If you've ever tried typing a post on a forum or elsewhere from the iPhone, you've discovered that editing what you've typed is a near impossibility if the text entry box is too small, as two-finger scrolling doesn't work.
By tapping/holding inside a form and bringing up the magnifying glass, you can scroll up and down by dragging and holding the magnifying glass in the direction you wish to scroll.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20080827084059882