Address Book plug in to Automatically address envelopes in Create

Place the folder "Address Book Plug-Ins" into your Library Folder, and relaunch Address Book.

Now, when you hold the mouse down over an address (such as the word "Home" or "Work" in the Card view), you'll get an option to address this person, and it will put your own address on the envelope as the return address.

Give it a try!