This Ars article from last week gives some context and explanation:
http://arstechnica.com/business/news/2012/02/critics-slam-ss...
In short, at least one CA, Trustwave, has issued a "subordinate CA certificate" (which allows another party to issue certs which will be trusted as if they were from Trustwave) to a network admin who used it to create forged certificates and intercept SSL traffic on their internal network.