The attack was discovered on March 29 by security firm WebSense, and the injected domain was called lizamoon.com -- thus, the name of the mass-injection is "LizaMoon." According to WebSense, LizaMoon uses SQL Injection to add malicious script to compromised sites. While the first injected domain was lizamoon.com, additional URLs have since been injected in the attack (WebSense has a full list here).
The method of using an injected script redirects users to a rogue AV site, which tries to get people to install a fake anti-virus program called Windows Stability Center.
When WebSecurity discovered the attack on March 29, 28,000 URLs had been compromised. The number quickly grew to 226,000, including many iTunes URLs (though the malicious code is neutralized by Apple).
"The good thing is that iTunes encodes the script tags, which means that the script doesn't execute on the user's computer," WebSense security blogger Patrik Runald wrote on Tuesday, "So good job, Apple."
The number of infected sites now appears to be over 1.5 million (as of April 2 [US], a quick Google Search shows 1.53 million infected URLs) -- but WebSense is quick to point out that a Google Search is an inaccurate metric. Google search spits back unique URLs, not unique hosts. Thus, there are likely less than 1.5 million infected sites, but WebSense says it's safe to say that the number is in the hundreds of thousands.
The attack continues to rampage across the Internet, and currently doesn't show any signs of slowing down. So don't install any web-based anti-virus software that claims your computer is full of bugs.