Spam/abuse deterrent: Timed form

This isn't an original concept; I read it somewhere, can't remember where now. But I don't know that this is implemented in Drupal anywhere and am considering a small module to do it. I'm looking for feedback along the lines of:

• Any reason why this wouldn't work?
• Is there already something that does this?
• Can you think of any roadblocks?

Basically, this is yet another technique to discourage the many bots that try to create accounts at Drupal sites, or hack contact forms to send spam email to members. The technique would probably work for other kinds of forms, but those are the two biggies.

The technique stems from the observation that most bots fetch and submit forms really fast. While a human will typically take seconds, or 10s of seconds, to complete a form, a bot will generally do it in under a second. So basically, the solution is to not accept a form that's submitted "too fast."

This won't completely deter bots, of course, but it will slow them down.

Here's the basic idea of the module:

1. Present an admin interface that lets the admin specify a list of form ids, and a minimum process time.
2. Use hook_form_alter() to attach a '#value' item to each form of that type, where the value is the time the form was served.
3. In the form validation, throw a form error if the form is submitted too soon after the serve.

The error won't wipe out the form, so a human could wait a few seconds and resubmit. Like I say, this won't completely block bots, but it would slow them down.

Another variation would be to keep track of the IP addresses of the submitters, and only allow one form submission per N seconds from a given IP. This would be more work, but would reduce the possibility of parallel submissions from the same bot.

Thoughts, anyone?