Once you have a legitimate certificate, pulling off a MitM attack is as simple as proxying the traffic through your own system. If you can trick the user into hitting your server instead of the legitimate server, *cough*DNSPOISONING*cough*, you can impersonate the legitimate server via proxy, and log everything the user does. And the only way the user can tell is if they actually look at the IP they're hitting. How many people do you know that keep track of the IP of the server they're trying to get to?
Surely there's something that will prevent this, right? I mean, the fingerprint of the certificate has changed, so the browser will tell me that something is amiss, right? Well, actually, no. In fact, if you replace a valid certificate from one CA with a valid certificate from another CA, the end user typically sees no change at all. There may be options that can be set to alter this behavior, but I know of no browsers that will detect this by default. Ultimately, this means that if an attacker can obtain a valid certificate and redirect your traffic, he will own everything you do without you being the wiser.
And now, just to make things more interesting, we have this little beauty.
This is an SSL interception device sold by Packet Forensics. In short, you provide the fake certificate and redirect the user traffic and the box will take care of the rest. According to Packet Forensics, this box is sold exclusively to law enforcement agencies, though I'm sure there are ways to get a unit. For "testing," of course.
The legal use of this device is actually unknown. In order to use it, Law Enforcement Organizations (LEO) will need to obtain legitimate certificates to impersonate the remote website, as well as obtain access to insert the device into a network. If the device is not placed directly in-line with the user, then potentially illegal hacking has to take place in order to redirect the traffic instead. Regardless, once these are obtained, the LEO has full access to the user's traffic to and from the remote server.
The existence of this device merely drives home the ease with which MitM attacks are possible. In fact, in a paper published by two researchers from the EFF, this may already be happening. To date, there are no readily available tools to prevent this sort of abuse. However, the authors of the aforementioned paper are planning on releasing a Firefox plugin, dubbed CertLock, that will track SSL certificate information and inform the user when it changes. Ultimately, however, it would be great if browser manufacturers would incorporate these checks into the main browser logic.
So remember kiddies, just because you see the pretty lock icon, or the browser bar turns green, there is no guarantee you're not being watched. Be careful out there, cyberspace is dangerous.