reCAPTCHA on a commercial / high traffic site

We're planning on using reCAPTCHA as a throttling/gateway/security mechanism on an otherwise open registration site that could potentially see upwards of 20-30K concurrent users. I can not find a link to the latest reCAPTCHA terms of service, and do not wish to violate any rules. Is there a limit to the number of reCAPTCHA requests that can originate from a single api key? Are there any other restrictions? Is there in an SLA or other commercial support available? Thanks. -- Mo -- You received this message because you are subscribed to the Google Groups "reCAPTCHA" group. To unsubscribe from this group and stop receiving emails from it, send an email to recaptcha+unsubscribe@googlegroups.com. To post to this group, send email to recaptcha@googlegroups.com. Visit this group at http://bit.ly/1dkFnYd. For more options, visit http://bit.ly/P65DvS.