Like I said yesterday, it seems like a greater number of sites are being hacked and injected with malware and/or spam. Heck, I just reported this morning that Green Party was hacked into and removed from the Google index.
Google recently posted a blog post named Best Practices for Verifying and Cleaning up a Compromised Site offering three tips.
- Verify Your Site with Google Webmaster Tools
- If Your Site Has Been Compromised, Perform a Comprehensive Cleanup
- If You Switch Hosting Providers, Disable Access to the Old Version of Your Site
Of course, the blog post goes into more detail on each of these steps. What is interesting is that I never heard of the last point before, but it is an excellent step. You want to keep the old site live as possible on the old server, but you definitely want to secure the site as best as possible.
There is a thread on this topic at Google Webmaster Help, unfortunately he kept it closed to questions, so no one can ask questions about that post in that thread.