That is why the CA-system is a joke, you only need to compromise any of the CA's that are trusted by default to fool all certificate users.
The big problem with this is that even though nobody knows your private key, they don't know your public key either, they have to rely on knowing the CA's public key and accept whatever public key they get when they connect so long as the CA says that the key is kosher. So let's say the NSA wants to track what you're doing on yourdomain.com, they just send a secret court order to VeriSign saying that they must turn over their private key that they use to sign certificates because computer security is a terrorist threat. Now they just use their own public key and send you a cert signed by VeriSign saying that the NSA key is the correct key to use. Because the NSA has more or less priviledged network access they can intercept traffic going to the IP address for yourdomain.com and just do a standard MITM but replacing the good SSL cert with their bad one.
What complicates things further is that there are tons and tons of CAs out there that are trusted by default and there's no isolation (AFAIK) such as "only these 10 CAs can sign certs for .com" so when the Iranian government wants to dish out some Orwellian justice on it's citizens they now have that huge mountain of targets to choose from because getting into one CA in the world (like DigiNotar) means that they can essentially break vanilla SSL until that CA's public key is blacklisted and all of the clients are aware of the revocation.
Just as a disclaimer, this was rather simplified but I feel like that's close enough to get the point across.