I just happen to notice that every single "person" I currently see in the birthday forum is a bot. Personally, I don't see what's the point of greeting a scripted account with a "birthday".
Lets be honest. The anti-bot text verification sucks ass. Why not make logical registration questions instead? Like what's (9 + 1)^10 or what do you get when mixing Red and Yellow? Spoiler: 100 and Orange
PHPBB3 has an implemented feature for this. Not sure about SMF though.