I'm not sure if this existed in older firmware, but in 1.1.1 when you're looking to type a new URL into Safari or Google Maps where the old URL is showing with the X button at the far right, you don't have to hit the X. All you have to do is start typing and the text field will automatically clear for the new text.
Nothing huge, but it'll save a second or two, right?
[robg adds: I can confirm that this works on the 1.0.2 iPhones as well.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20071003192820475