I just set up Gmail's new fetcher feature, called "Mail Fetcher" or "Get mail from other accounts". It acts as a POP client, rertrieving POP3 email using your password from any account you want. If you can prove that you own that sending address, you can then send email as that person. Gmail has had this latter feature from some time. In fact, you could simulate the whole thing by simply setting up a forward on the pop account in question. The new POP client feature just makes it a little easier to label emails as being work-related or personal-related.
Just had a question for the community. Does anyone else use this feature? If you do, have you figured out how to make Gmail check your email with a particular frequency? It seems to be set to an hour now, which is painfully slow.