That suggestion you have about the input form has merit. In fact, you don't even need to put a value in it. Just leave it blank by default, and it if gets filled in, then abort the registration. A few months ago, I ran a honeypot board which had 5 hidden form fields. They were in a tag marked as hidden, so the only way to see them was to view the page source code. Every bot which signed up filled out these hidden fields.
Ooh, that's interesting - so the bots are parsing the registration for form fields.

The div idea sounds better. Have a secret field and if it's filled in then it's a bot. Also have the input type=hidden to catch bots that aren't as smart (who don't parse the form).

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