This post is back to the good old-fasioned speculation based on evidence that I love so much.
You know how we figured out Glenn's last name? It was given with the Season 3 casting photos. I was casually looking through these photos when I noticed something. Why was Glenn given a last name, when characters like Andrea and Michonne are just Andrea and michonne (respectively)? What could glenn be a part of that might force him to reveal his last name? This points to the obvious conclusion that Glenn says his name aloud in his wedding. The evidence pointing it to season three is that whoever put together the cast photos for season three on the website must have been trying to make the names as accurate as possible. this person (A writer or producer, perhaps) might remember a scene they watched get shot, where a certain asian character married a certain Irish character and said their last name. RHEE! BOOM! EVIDENCE IN SPECULATION!
(to all the people who've been posting recently, we appreciate the contribution, but lets try to consider the relevance of a post before putting it up.)