The title pretty much says it all, with a tip of the hat to John Gruber's tweet on the subject:
If you select a portion of an email before replying, only the selected portion is quoted in the reply.This is how Mail on the Mac behaves, and is a welcome change in the new iPhone 3.0 OS. This tip is not mentioned in the updated iPhone User Guide, which you can download from Apple's iPhone Support Page.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090618053049164