I’m using WordPress Hashcash formally known as Spam Stopgap Extreme and I haven’t had a single comment spam in my moderation queue since I implemented it. Plus all legitimate comments are getting through without any problems.
The import-b2.php doesn’t work properly for some reason. I have a tutorial on my blog for switching from b2 to WordPress – http://www.wild-mind.net/archives/2004/09/12/changing-from-b2-to-wordpress/ – hope it helps you make the switch. You are going to love WP!
There is current info on the PHP attacks here – http://isc.sans.org/
There’s more information on this here – http://wordpress.org/support/7/19285
