I have multiple addresses associated with my IMAP account (ebay@, groups@, shopping@, etc.). When sending mail from my iPhone, the addresses associated with your account can be selected by tapping on the From field, and choosing an entry from the pop-up rolodex.
Unfortunately, while the iPhone's Mail preferences app supports multiple comma-separated From addresses in the account information (just as OS X Mail does), there is no easy way to add them. Any multiple addresses that were present on the computer when the iPhone was originally set up will have been added at that time. However, when you go to the Address field of your account, only legal SMTP characters are available for typing input, so there is no way to type the comma separator.
Now under iPhone OS 3.0, copy/paste to the rescue! To create your first additional address, just temporarily edit the Description field immediately below Address. This is a full-text field and you can enter your new address with a leading comma and space. Then Copy the entire string out, select an insertion point at the end of the Address field, and Paste the new address in. Exit Preferences, go to Mail and create a new message, and tap on the From field. You will see the rolodex pop up and be able to choose your alternate name. Voila!
After having created one additional address, adding others is simple as you can just Copy/Paste within the Address field without disturbing your Description.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090814134006673