Nope.

Fearmongers and doomsdayers have been predicting end times since... Well.. The beginning times.

For all descriptions of Planet X etc, it would be very visible.

In addition to not seeing anything like that, in infrared, or otherwise, we do not see its Gravitational Effect either.

Now, 20 years ago you might have been able to make a case. (Ok, not really but I'm being nice...) But now, in 2008 with 2012 only a few years away, there's no way we could miss something that big this close.
I say close because for the time limit- it would need to be pretty close to Earth to reach us by 2012.
Yet we have no trouble spotting Pluto or Eris or countless other tiny objects that are much further away.

You can start here, and if he hasn't by the time I've finished this post, 01100101 will undoubtedly produce this absurdly long list of threads related to the Niburu/Planet X nonsense.

ETA: YOu will note that it says 2003. That is because planet X was supposed to kill us all in 2003. Nothing happened of course.
So they extended it to 2012- especially relying on a misconception about how the Mayan calender works ( It rolls over in 2012 just like every other calender rolls over at some point). I wonder what they will say in 2012 when nothing happens?