Project Honeypot offers an API you can call.

If you are looking for a universal, comprehensive, up-to-the-second database of all bad IPs everywhere, stop looking. It doesn't exist. All you can do is accumulate ranges. No two people have exactly the same rules for what's acceptable and what isn't, though some areas have a pretty clear reputation as bad neighborhoods. (Like real life.)

Now, if I suddenly hear about a WebmasterWorld member retiring to a private tropical island at the age of 37, I may withdraw the preceding paragraph :)

@ rollinj

If you want one to help you put the brakes on quickly just PM me. I have a list of about 150 lines of CIDR IPs that weeds out most problem hotspots throughout the world. I will say right here though that it will ultimately be your own responsibility to verify them that you aren't blocking something you might not want blocked. But, as far as I know there are very few, if any ISPs blocked other than whole countries like China, Russia, a few Eastern Europe countries, as well as large portions of south Asia.

Most of the server farms throughout the world are also included in the list. But again, nothing is identified, you'll have to look them up yourself.