no its only built in for page comments but have alook at the code, its pretty easy to integrate / add your own spam check. Or if you want to try out ingos recaptcha (http://recaptcha.net/) module check out http://open.silverstripe.com/browser/modules/recaptcha/trunk

I tried implementing the recaptcha module, but couldn't get it working.
I downloaded the trunk files, uploaded them to the 'recaptcha' directory in my SS root directory, added my public and private API keys in the _config.php file within the recaptcha directory, and did a db/build?flush=1. What am I missing?

The module just provides the FieldType class for it. It doesn't automatically insert it in your forms. In your form code you need to tell it you want to use it. Depends how you build your form but basically you need

new RecaptchaField('Recaptcha');

In your list of fields