The blacklists from Google’s Safe Browsing Initiative (where Safari checks for 'fraudulent websites') are contained in a database cache file called SafeBrowsing.db - the file was created when you first launched Safari 3.2, and if you have the browser open, the file is modified approximately every 30 minutes.
In other words it is part of Safari's (version 3.2 onwards) new anti-phishing security feature.
As an alternative to turning off 'Warn when visiting a fraudulent website', which will lose you that important security feature, you should delete that database file, (but first close Safari):
Home/Library/Caches/com.apple.Safari (this is a folder)/SafeBrowsing.db
It will be recreated next time you open Safari, and will then start again collecting details of dodgy websites.
If you are interested:
How the Anti-Phishing feature of Safari 3.2 works:
http://www.macworld.com/article/137094/2008/11/safarisafebrowsing.html
Some users have mentioned that the latest security updates 2009-001 and 002 cured the problem.You must also update to OS 10.5.7.