Usenet newsgroups: news.admin.net-abuse.email. Just go to google and search that group for Rackspace.
They're slow to terminate spammers and/or their websites. Currently the more rabid anti-spam groups(The ones who will list all of the IP blocks from an ISP instead of just the IP addresses of the spammers) have listed most of Rackspace, and if you're there, and not being blocked, it's only a matter of time before it does happen