If you have secondary email addresses tied to your Gmail account, using one of them to “send from” on your iPhone and/or iPad is not a straightforward process. Nick Cernis did the research and figured out a way to make it work, although be prepared for a somewhat tedious process to make it all happen:
The short version is that you just list your email addresses separated by a comma in the Address field of the mail settings on your iPhone/iPad. Sounds simple, but it turns out to be important that you follow certain steps in order, and that you set your Gmail address up as an IMAP account, not a Gmail account. If you’re already utterly confused, read on as I attempt to deconfuddle you.
Nick does an exceptional job breaking down each and every step, presenting screenshots to make it as user-proof as possible. Although nobody has mentioned anything of the sort, I really do hope iPhone OS 4.0 solves this problem. There should not be a need to jump through this many hoops just to setup additional “send from” addresses on the iPhone and/or iPad.