Do you have any links on how to do this as when I've searched I end up with very conflicting information - some say yes, some say no.
Note that this only works for gmail accounts, or your own domain if you use the Gmail for your domain service.
First, go into your Gmail account and turn forwarding on. Forward all of your mail to your MobileMe account.
Then, set up your iPhone and Mail.app accounts to receive mail from your MobileMe account.
Finally, go to the advanced settings in both iPhone Mail, and Mail.app, and make sure you use Gmail's SMTP server. You will need to use your Gmail address and password in the server settings (the server address is smtp.gmail.com).
Now, anything sent from your iPhone or Mail.app will have your Gmail address as the "from:" and "reply to:" addresses, because Gmail's SMTP server automatically changes that information on the outgoing message.
Again, this will NOT work from the MobileMe web app. You'll still need to use the GMail web app for webmail.