I haven't seen this documented anywhere, but I discovered that you can select an entire paragraph of text by quadruple-tapping on it when entering text in the iPhone. For instance, when replying to an email, you can quadruple-tap on paragraphs in the quoted reply, then cut them.
You have to do it pretty quickly, and take care not to move your finger too much between taps. Try it a few times, though, and you'll get the hang of it. And yes, I know it might sound weird, but I actually find it quite useful sometimes.
[robg adds: This works, and isn't covered in the iPhone user's guide.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20100312090805523