@paul - Yes, we can add as many accounts as we want - you are correct.
The problem, is that I have Mobile Me and when I access it via a browser, all of my email is in one place and I can respond from and write from any account.
On the iPhone and iPad, I can only send/write from the MobileMe account.
Adding multiple accounts creates separate inboxes and duplicate emails.
It sounds as though this feature is supported on Mac's and online, but not on iPhone and iPad.
I have had that problem, actually: with Gmail. I had one account handling all my different address, but to reply with the address the email was sent to, I had to either use the browser version or set up multiple accounts. (Or deal with the multiple email issue, which I hated.)
One idea is to use the browser version, create a homescreen bookmark for it, and replace the mail app in the dock with that shortcut. But with the "All Inboxes" now standard I haven't an issue--I decoupled the one account I really needed the "reply as" to work on and now have two accounts on the phone. If I absolutely HAVE to use Reply As on another account I just use the browser shortcut.
One last idea: have you tried setting up accounts, then disabling them without removing them? For some reason I remember doing that and still having the option to use those addresses as reply as options.
Sorry none of these is a true solution.