This doesn't appear to be covered in the manual, so I thought I'd pass it along. When you're in the iPhone's text entry mode, if you hold the Delete key down, it will delete letter-by-letter for a few seconds, then it will begin deleting entire words.
This is great when replying to e-mails and you want to trim out the unrelated content.
[robg adds: I wouldn't describe the process as "quick," but "quicker than letter by letter." Being able to select a chunk of text would be the best solution; one press of the Delete key would then make it vanish.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=2007070511095429