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, 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) 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 could delete that database file, (but first close Safari:
In Tiger:
Home/Library/Caches/com.apple.Safari (this is a folder)/SafeBrowsing.db
In Leopard*:
Private/Var/Folders/AF/AFONO9KnGpijQuAcxb6vf
++TI/Caches/com.apple.safari/Safari/SafeBrowsing.db
In Snow Leopard*:
Private/Var/folders/iI/iIaUOKJyFS0xmSIANVYiD
++TI/-Caches-/com.apple.Safari/SafeBrowsing.db
(*This location may vary depending on your OS. Also, the name of that weird sounding folder will be unique to each user.)
You can get to the Private/Var folder by using Go To in the Finder menu.
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 onwards works:
http://www.macworld.com/article/137094/2008/11/safarisafebrowsing.html