Hmmm, perhaps this is the first of many in a dystopian future of human CAPTCHAs... "Please solve this equation in 200ms or less. Sorry, no humans allowed."
Actually, that's a fascinating idea that could be put to a different purpose.
Spambots try to spew spam as quickly as possible, right? I wonder how effective it would be to flag those who fill out the signup forms too quickly as likely spammers and check up on them or impose extra limitations until they had been using their account for a while without spamming?
