I created a bogus account called Select Account. Then, In the Composing section of Mail's Preferences, I set the 'Send Mail From' pop-up to the bogus Select Account. I then went back to the Accounts section, highilighted my bogus account, and on the Advanced tab, I unchecked the Enable this Account setting.
Now, in my In box, the bogus mail account does not show. However, when I create a new message, the Account drop-down shows this bright red (because it's disabled) Select Account at the top of the list. If you don't select an account and try to send the message with your bogus account, you get an error from Mail which allows you to select an account.
Just a note: if you visit the Composing section of Preferences again, Mail will unselect your bogus, disabled account. You then have to enable the bogus account, select the bogus as your default, and then disable the bogus account again.
How nice is that? No more sending "let's play hooky today!" notes with the work email account!

