"So I spent 10 minutes writing a little script that checks for mouse movement and clears a pre-populated field. If the field isn't empty, bot it is"

Nice idea. I tend not to use the mouse a whole lot once the 'reply' link has been clicked, have you had any complaints of legitimate posts being lost?

I'm wondering if adding a check for key down/up events would mitigate this potential issue since a spam bot is not likely to generate those either.