Thanks Shawn for taking the time to write an in depth article.
This reply is for info share only.
Seems there are differences in effectiveness between SMF's V2.0.x Anti-Span Verification Questions and the SMF MOD Anti-Spam Verification Questions for SMF 1.1.7 found at http://custom.simplemachines.org/mods/index.php?mod=1516
Background: My club's SMF forum was under increasing foreign registration attack since March 11. IP Bans failed to stop the registrations and BotScout helped slow the flow but did not halt the attacks. Finally in August 2011 I installed SMF MOD Anti-Spam Verification Questions for SMF 1.1.7 with three questions. How many sides to a triangle, How many sides to a square, What is the sum of 3+4. I had peace at last. Implementing that mod stopped all foreign registration attempts.
Fast forward to today. I upgraded the club SMF forum from version 1.1.15 with the Anti-Spam Verification Mod listed described above, to SMF version 2.0.2
I implemented the same three questions with the built in anti-spam verification features of SMF V2.0.2.
Within the first 12 hours under this configuration, there were sixteen successful registration attempts.
All User Registrations require Admin Approval, so I get to delete these Foreign Registrations. No impact to our members.
I just find it peculiar that the Anti-Spam MOD used in SMF V1.1.15 was 100% successful but the same verification functionality using the same questions in SMF V2.0.2 has a crack in the armor.
Just wanted to share my experience.