Sometimes I had to create AT that may be instantiated by anonymous users. This product provides a dummy field that allow to request captcha validation (through collective.recaptcha) when a new object it's created.

Just install the product; then in the plone_control_panel you will find the new entry "Select type to validate with captcha". Here you can select types you want to relate to captcha validation.

Remember to configure collective.recaptcha here: /path/to/your/site/@@recaptcha-settings

Then, creating a new AT configured to deal with captcha validation, you'll be able to see captcha field at the form bottom

• Plone 4.2 (Not tested on older versions)
• collective.recaptcha
• archetypes.schemaextender (to extends AT dinamically)
• plone.app.registry (to store settings)