I have two mail accounts - work and personal. I'm always sending from the wrong account, since Mail seems to pick which account I'm sending from somewhat randomly. However, I've found something that helps (even if it's not a "real" workaround). In Mail's Preferences -> Composing -> Addressing section, there is an option "Mark address not in this domain:". If you put a domain name in there (like my work account's domain, say, atwork.com), all addresses NOT using that domain will be marked in red - including the "From" account!
So now, when I send email, I have a visual cue - either all the addresses should be red (personal mail sent from my personal account), or none of them should be red (work mail sent from my work account).
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031104001456983