I forward a few different email adresses to my gmail account, and one of my biggest gripes has been that I've had to manually change the From address when replying to an email received via one of the "other" accounts.
So I mentioned that in a comment over at Lifehacker, to which another commenter replied that there already is such an option available in the settings.
And sure enough, there is. Apparently, sometime between the-last-time-I-checked and now, they've added that option, but I simply haven't noticed. Bastards. Absolutely lovely bastards, but still.
I wish they had told me.