Nah, it's no secret.
Carl Spock began in the TOS Caption Contests a number of years back when one of us decided to Photoshop a big '70s afro and goofy period glasses onto Leonard Nimoy, creating a new (and very groovy) character that was quickly named "Carl Spock." The meme took off from there and Carl has made countless appearances in the BBS both in caption contests as well as in other threads all over the board.
He's not nearly as popular or widely posted as he used to be, but I like to keep his spirit alive by using him in a few of my avatars.