Gmail: Associate Addresses to Interlocutors

I have a number of different email addresses that all wind up in my gmail account. Some of them are personal, some business. Gmail is smart enough to use a particular email address in your reply when someone writes to you with it, but it would be nice if it went a step further and looked at what address you normally use with a given person, and defaulted to that, so that whenever I write person A, it uses the business address, and switches to the personal address when I write new emails to person B.

I guess that’s one of the problems with relying on software you don’t have the source code to.