On the Mac, when you are replying to an email you can highlight text in the email before hitting the Reply button and when you do hit the button, your reply will only contain the text in the original email that was highlighted. This is really convenient if you are trying to reply to a specific question in an email.

Now that the iPhone (3.0) has cut-and-paste capability, you can do the same thing on the iPhone mail app. Just highlight the text you want and then hit Reply. Since it's a mobile device, this is especially helpful if you are replying to a long email.