Mark I haven't emailed you in over 5 years. I have no idea what the address is.
Tagging people in posts is a real easy system to make. I programmed something like that a few years back. I was pretty simple with mine though. You have to surround someone's username in the @ symbols before the system can pick it up. Something like @testuser@
Then it would spit it out into someone's sidebar.
Translated to this site it would seem accurate to do what Flux said.