I'm pretty sure you(uber) have noticed a few bots that appeared on this forums, thanks to it's growing popularity, you should consider some actions, such as the usage of openID services such as STEAM, facebook openID is long compromised and should be avoided, I'd recommend you to use STEAM openID since that is the one that seemed to block spam the most efficient way. Alternatively, you could implement a question system in the register form as well. Just please make sure you stop the bots before they become a real problem.