Just to clarify, the issue was the ad network adbrite/blacklabelads allowing malicious code to run across its network. This only affected a fraction of the pages, and only ads that were served to guests (i.e. not logged on). What specific malware, I have no idea, but basic preventative measures are likely to be effective (not using a very old browser, keeping your flash and java up to date, not saving or running any .exe files you get served by ads etc. etc. etc.).
Googlebot crawled a-o, found the offending pages and emailed me (as the site admin) to inform me that it had found offending code served by adbrite and that it had blocked a-o in the manner you all describe. I then removed the adbrite code immediately and submitted a recrawl request to googlebot. googlebot crawled again, found no malware and removed the block.
All within about 12 hours.
now we just need to find an ad network that doesn't serve malware...
