Nah, I don't think Guybrush ends up in our dimention. Because after the first stab, he's still in a world with a zombie with a voodoo doll, and Chuckie's eyes still glow in the end. I'm thinking more in the direction of a dimention that isn't bound by the laws of nature, where the world turns out to be the way it is because Guybrush doesn't want it to be so. If another person got sent to the same dimention, he would probably experience a whole other world than Guybrush.
Ash735: I liked the theory of the whole underground tunnels-thing being a trap, where everything is put there because LeChuck wants you to do it. Probably some kind of illusion. The question is what kind of illusion, and where it starts.