I cannot believe it has taken this long for me to figure this out:
Multiple identities in one account with Apple Mail.app (link)
I work with an average of five different email accounts to separate work from personal from junk. Essentially, all of the email addresses I use pour into one single account except for those associated with email lists (because of the sheer bulk of messages I would receive and can't process). The problem is that I thought Apple's Mail.app didn't have an easy way of sending from these "identities" unless you added the identity as another mail account. Not true. If you follow the instructions in the above link, all you have to do is take the one account you wish to change and add every email address (i.e. identity) you want to send from in a comma separated list in the Email Address field where you set up your account. I know this works on OS X 10.4 (aka Tiger) and will test on 10.5 (aka Leopard) later. Props to Jonathan for finding this very simple solution.
