VOTE: emacs.

My dad showed me to just knock on the wall, moving my hand horizontally over the wall as I rap my knuckle against it a few times per second. It makes a higher pitch and a more solid, "hard" sound at the studs than over open drywall, which sounds hollow. Then to confirm, drive in a finishing nail, halfway so you can…

One other thought I just had: if you have access to Flash, can you just put a small flash movie in place of the element? Do spambots read flash content somehow? I know they use character-recognition programs to crack images of emails.

Off the top of my head, I'd write a javascript function that builds a html element containing the right characters. If they weren't in the right order, for example using some simple arithmetic function to reorder the characters in an array, wouldn't this be enough? Do spambots run javascript on the pages they read?