Popup on LiveJournal
The idea is to create a flash banner ad that asks a flag from malicious server to decide whether to do the popup. During the checking time, the malicious server will say no to it. So, it will pass the check from creative validator. Once the ad is live, the malicious server will say yes to it and bad popup will go wild.

Doubleclick serves up vast malware blitz
A German marketing company called AdTraff that submits problematic creatives to the ad network. This creative (.swf) will do the forceful redirect to malicious site that will automatically scan your box to identify issues and push you to buy their software. Inside that Flash file are encrypted redirects to whatever site that Adtraff is pushing. The trick is that the redirect data in the Flash file does not show itself when the creative people at the website upload it. The redirects are triggered by times, geo location, etc.

Malicious content appearing on 123greetings.com as discovered by seo.mhvt.com