We are getting hardly any spammer signups these days. The last one was from a few months back, I think, at which point Shevron and I tightened security a bit more and haven't seen one since.
We actually are using a rather simple combination of a few blocked spammy email domains and custom WoW-themed challenge/response security questions. For example "Name a playable race in WoW", which is pretty much impossible for a bot but should be utterly trivial for someone who wants to sign up on the forum of a WoW guild.
Also, we are using 3.0, not 3.1.