For Jan-2011, the number is pretty standard, but a little bit lower than previous months. Out of those top 1 million sites, around 1.4k had their main domain blacklisted (1,447 to be more exact). Compared to previous months, you can see a decline in there (2.1k in Dec, 2.5k in Nov, 3k and Oct, etc).
Why the decline? I would hope that it would be related to a safer Internet and people taking security more seriously, but based on the latest malware we have been analyzing, it seems that they are able to hide from Google pretty well (by not displaying the malware to Google’s IP addresses or to their user agents, etc).