I've come across a site, www.zetatalk.com, which talks about the passage of a planet known as the 12th planet, and is referred to as "Planet X" in other places. What's the deal about it?

::)
There is no scientifically verifiable, hard evidence for the existence of Planet 'X.' Although some 'facts' have been presented, none of them have been verified under the scrutiny of established scientific investigation.

There is a lot of information about the so-called Planet X at badastronomy.com

Are you talking about exoplanet HD 189733b?