I have a installation of Multisite that has a history of serious signup spam problems.
When we ran WPMU 2.9.2 we tried this .htaccess method, this math plugin, and CAPTCHAs. After little success with those options we added some common email domains to Limited Email Registrations. We noticed that even after adding this measure we were still getting signups from domains we had banned. This made me think that there was a hole somewhere in the signup script (explaining our limited success with other measures). Are there any known issues with Limited Email Registrations or any other holes in the signup process with older/current versions of WP Multisite?
We have since updated to 3.3.1 and continue to experience significant spam signups. With the .htaccess method, math plugin, bad behavior, and of course e-mail activation, we still get 20-50 spam signups a day. I have a hard time believing that their isn't a hole somewhere.