If you are working with different email Address in Gmail... now you have a new option:

Go to Settings -> Accounts

>When I receive a message sent to one of my addresses:
> * Reply from the same address the message was sent to.
> * Always reply from my default address

This is almost (but not quite exactly) what I want.
I have a number of addresses forwarding to my gMail account. The trouble is I don't want that "On behalf of" thing, all I want is them all outgoing mail sent from megmail.com with different Reply-to addresses.

gMail will only allow you to add one account with the email address megmail.com...

Splasho.... that is exactly what it used to be before they introduced the whole "accounts" thing. now, it is in a setting if you click on your @gmail.com in the accounts setting
click "edit info" then "Specify a different "reply-to" address"

Sorry, I wasn't clear.. I have that set up, the trouble is that I have 3 different reply-to addresses which I use for different things..