Well it's not like we included something in 1.6.4 to tell all the spam bots to come to your forum, it'll be sheer coincidence.
Yes, both my forums are being spammed, how I hate this bots...
Today I installed the recaptcha plugin. I hope this fix the problem. If not, I will add a security question.
Great info in this thread - Thanks everyone
I think the increase in spammers is coincidence aswell - I wasn't hit at all for a long time, then last week I got smashed