My phpbb forum gets around 2000 uniques a day and used to attract about 30 bot signups a day too. I spent ages looking around at overly complex mods and solutions, and then I found something called the vip mod, which simply adds an additional field to the registration page, the user just has to enter a specified number and thats it. Since then not a single bot has been able to signup. Try it out, takes about 5 minutes to implement... I'm not allowed to post links so just type the following into google and click the first result in the list... myvipcode.zip