Leif wrings his hands:
My head almost went all Scanners while typing the “a @gowalla.” Strunk and White, where are you when I need you the most? Is it “a @” or “an @”?The correct way to do this is to keep the "@" silent, and I'll tell you why.
Let's take Erica Mayer's tweet
My buddy @jonberrydesign is at the #Olympics working in graphic design! You should follow him, he's a swell dude <3Do you pronounce this, "My buddy at-jonberrydesign is at the hash-Olympics"*? I don't. When I sign up for Twitter, I pick user names that don't involve "at" being said at the start. My Twitter handle is Punsultant. I am not the at-Punsultant; nor am I running the at-MNBeardOff.
The "@" indicates that the string of letters immediately following it is a user name. Similar symbols include the apostrophe, which indicates possession or acts as a replacement for letters within a word; the period, which indicates the end of a sentence; and the quotation mark, which indicates speech from someone other than the narrator. These symbols are called punctuation and are not pronounced.**
My point is this: "@" is punctuation that Twitter has invented. Find me a reputable style guide that says punctuation should be pronounced and I will start saying "at-username." Until then, the "at" should remain silent.
*I would like to compete in the hash Olympics.
**Sometimes quotation marks are pronounced when reading aloud. I don't care.