I saw this posted anonymously on the
MacNN Forums, but I wish I could give credit where credit is due. If you have ever wanted multiple addresses to be associated with one email account (which I have) so that you can send mail from any number of addresses without resorting to creating many "nearly duplicate" accounts, simply enter all the addresses separated by commas in the "Email Address" field of the account setup dialog.
I tested this and it worked beautifully. Still, I wish I could just type in any from address at composition time, as I have a wildcard email that I use to track which sites sell my email to spammers (i.e. myname-buy.com@mydomain.com, myname-sports.com@mydomain.com, myname-evilbrother@mydomain.com).
Hope this helps out some folks who thought Mail.app had a limitation that it apparently does not have.