Using Mail on OS X, you can assign multiple sender addresses to one account. As covered in this ancient hint, this is done by just separating each address with a comma in the account's setup screen.
This same trick now works with the iPhone 2.0 firmware -- before it would just use the first address in the list. So if you have, for example, a Gmail account where you forward all your other mail addresses, you can now easily send mail from all of those addresses. Of course, your SMTP server must allow sending from foreign addresses for this to work.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20080724122539850