I stumbled upon this: http://www.stopforumspam.com/ just now and wondered if it has been considered (or is even known about) for the StackExchange signup procedure? Maybe not necessarily to block people from signing up (false positives?) but maybe to make more verification needed, or flag the user as a probable spammer?
I don't know if it would be wise to lean on an external partner. There is an API limit for reqeusts (I don't know the subsciption rate for SE, but again, leaning on an external partner that has obvious limits might be tricky).
Though a good initiative it may be, I'm not fully convinced the site you link to is a good solution for a community as large as this. I must say that the site itself is not a real big insentive to 'believe' in the product. Do you have personal experience with the product/site/API ?