Mail in Lion properly responded from the same email address that received the email. Mail in Mountain Lion does not and instead replies from the default email address, regardless of the setting(s) chosen in the preferences.
Generally, if you have multiple e-mail accounts, when you reply to a message, your reply uses the same account the message was sent to. But I’ve been finding that Mail does not correctly choose the account, and have seen a number of messages bouncing because the selected account is not a member of the mailing lists.
Update (2012-08-08): Joe Kissell:
However, in Mountain Lion, Apple significantly changed the rules by which Mail chooses the default From account for outgoing messages. As a result, if you don’t carefully (and manually) check each outgoing message, you might find that you’re sending messages from an unintended account. This can have serious consequences if, for example, you end up sending work email from a personal account or vice-versa.
Update (2013-03-14): This is apparently fixed in Mac OS X 10.8.3.