Thanks for your reply jestep.
I'm thinking too that it would have to be by domain and not by IP to be fair (yikes some IPs have HUNDREDS of websites sitting on them). But there seems to be mention of this going back to 2002 (I've done a search on Google for Google Blacklist IP).
The reason I'm asking is that a friend's host is blacklisted by SPEWS. Seems the host she is using has an upstream that is spam friendly? We were more concerned about search engines blacklisting her sites as she doesn't need the host for emailing newsletters or anything.
Check
http://www.whois.sc for your domains and read the blacklist status. That's how she found out it was listed.