I currently manage several email addresses through a single Gmail account, and apart from Google’s creative interpretation of the IMAP protocol, it’s been grand. I can send and receive messages to or from any of my accounts from a single, virtually spam-free, inbox. I wanted this ability on my iPhone as well but the setup is not as straightforward as it could be. Here’s what worked:
First, you need to setup a new mail account on your iPhone for Gmail, but not using the iPhone’s Gmail account type. Seriously, this is key. To be more specific, after you select “Add Account…”, choose “Other”. I should also mention that you can’t do this using Google Sync’s Exchange support. Now, after you’ve typed out your email address and password 12 times and filled in the other important bits as described here, you should have a functional IMAP email account. Which leads us to the second (not so) tricky part. The way you add multiple addresses to your new account is to input them into the “Address” field, under “IMAP Account Information”, with each address separated by a comma. Just like Mail.app! Except, you can’t type a comma in that field. I’ll wait while you try it… No? Good. Anyway, the obvious way around this is open any app that supports editable text, type in your address list, copy it, and paste it back into the “Address” field. You could also use the Description field. It’s up to you.
Now you should be able to select any of the email addresses you entered as the “From:” address in Mail.
UPDATE: Here’s a great article on setting up both Mail.app and iPhone Mail for Gmail.