Nuno was complaining about the lousy support of multiple profiles in Mail.app. I agree with him, but today I noticed that I already was using some obscure features that make it all easy :).
First, in the preferences account panel, you can put several email addresses, comma separated. They will all show up in the drop-down in the Compose window, so you can change the From address easily. With MailEnhancer, you can even change the signatures auto-magically.