I would like to see a plugin that adds a hidden custom field to the register page to catch bots and stop them when they fill in the empty field. Is this possible or even a good idea?
Well I'm glad it is easy as I don't know how to write php (yet) so I wouldn't be able to accomplish it although I've seen instances in other CMS' where it works pretty good to slow them down.
Nothing a real user ever see, its a hidden input. A hidden input is useful in the way, that automated bots tend to fill out every input, even hidden ones ( type="hidden" ), therefore you can extend your anti-spam with it. While its not bullet proof, its a damn good method