Now I'm no expert on anything about this, but I do remember reading about it.
The dispute is not over whether or not it can be orbiting the sun, but rather it's size. That's the whole issue over whether or not it can be a planet. What is that smallest an object can be and still be called a planet? I'm pretty sure that 'Planet X' IS within orbit to the sun, but I'm pretty sure it ISN'T another planet. If I remember correctly, it may be a large chunk of rock, or even a moon of Pluto.