How can I configure the iPhone to send/receive mail through Gmail, but using a different e-mail address?
This worked for me


  Assuming you have already setup the alternate account in your gmail 
  
  
  Remove the gmail account from your iphone  
  Add new mail account
  
  Settings > Mail > Add Account > Other  
  Enter standard gmail id and password in New Account screen  
  Enter standard gmail id and password in IMAP and SMTP screens (imap.gmail.com, smtp.gmail.com) 
  After account is setup, under "IMAP Account Information" change address field to the desired account (Note that the "Incoming Mail Server" settings will remain as 
  gmail info, this is the difference between creating the account manually and using the automatic gmail setup)
  
  
  
  Enjoy