In the old yahoo mail (not the new beta) go to options, mail addresses, add, enter the info, then back at the mail addresses page select the new account and next to sending mail click 'make default'pb - I have the $20 upgraded yahoo email account and still can't figure out how to set it up to send from another email address. I want to be able to reply through yahoo but show the reply coming from a none yahoo address - and I want to do this all on my iPhone. I can setup a pop3 for my corporate email but then I don't get the mail to push.
I have not been able to send from different address on the iPhone though. Using other mail providers (like aol) I can use aol's IMAP server and gmail's SMTP and it will receive from aol and send out from gmail. However since yahoo's IMAP server isn't public this can't be done (afaik, I'd love tone wrong on this one)