I have my work computer set up to also check my home email account with Mail. The problem is that I can't reply from this account at work. If I reply to a message sent to my home address at work and forget to manually switch the reply account, I have to hold the mail for later, go to the outbox, open the message again, and then change the account info. I've been trying for months to figure out how to set a default account for replies in Mail with no luck.
I finally discovered a very simple work around. Go to preferences in mail and select Accounts, then select the account that you can't reply through and click the Edit button. Once the edit field is open, go to the Email Address field and replace the normal email address for the account with the address of the account you want to respond from. It is that easy ... go figure!
[robg adds: I believe this works because Mail doesn't use the Email address to get your inbound mail, it uses the Incoming Mail Server and User Name / Password fields a bit further down ... and keep in mind this hint is for using the same Mail account on two different machines with a sending limitation on the second machine.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030226115915813