I just noticed a feature in Gmail that I haven't seen mentioned before. Under "Accounts" (in Gmail Settings) you can now choose which of your account address is used when you a reply to an e-mail. You are given 2 options (as I've copied them here from my Accounts Settings page):

When I receive a message sent to one of my addresses:
   0 – Reply from the same address the message was sent to.
   0 – Always reply from my default address (currently 'you'gmail.com)

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