I'm often frustrated by the fact that the ".com" key exists for entering URLs, but not when entering an email address in a form on web pages.
Now instead of pressing the ".?123" key to get the period (.), I just hit the space bar twice followed by the delete key, then type com. I know it's the same number of taps, but this technique is certainly faster. Note that this will only work on iPhones that have been upgraded to version 1.1.1.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20071008105233500