In mail versions prior to 6.0 you could into the accounts list and drag the accounts into order with the top being the account used as default. In 6.0 I can't drag the accounts around.I didn't eat my Wheaties this morning and can't find where you set the default outgoing account in Mail 6.0.
Ah, I found it. They still have it under Composing, Addressing, Send Mail from. Looks like a setting in previous versions too but I never noticed it.