PJ Vogt's Twitter grammar question: it's "an" except when it isn't

Today on On The Media's TL;DR blog, PJ Vogt asked an important question for our time: when using an indefinite article before a Twitter username, do you say "a" or "an"?

When I saw the link on Daring Fireball, I thought, that's an easy one, it should be "an," because Twitter user names start with "at."

But here's the Tweet that incited the discussion that led to the post:

This definitely sounds wrong in my head, because I don't mentally pronounce the @ for Guardian, perhaps because it is the publication's proper name, rather than a nickname or handle.

PJ doesn't really come to a conclusion partly because the a/an rule doesn't really change here: the real question is when do you pronounce the @? I propose that the @ should be pronounced (as "at") in instances where the username does not match the account owner's common name. I'm "at-tonyskyday" because no one calls me that in real life. Therefore, this is "an @tonyskyday" blog post.