First off, how did they determine it will happen in 2012? It's not like the galactic plane is paper-thin and we just go through it in a second -- it's quite thick and takes hundreds if not thousands of years. Other than that, I'm actually not sure where we are relative to the galactic plane right now and so I can't directly answer my own question.
As far as the other stuff is concernced, I think it's more that passing through a denser part of the Galaxy means that there's more of a chance of a close encounter with another star, which can then perturb comets towards us, which would again take thousands to millions of years to then get to Earth.
So besides the time involved and some of the reasoning, I'd say this idea holds water.
