I pointed out to posters on another forum, also alarmed about the earth passing through the plane of the galaxy that this not the case at all. The apparent effect is described as a result of precession, not a physical motion of the solar system. And as you point out, Grant, it happens at least twice per year as well as the earth orbits the sun.

The solar system is somewhat inclined in its orbit around the galaxy and so during its revolution, physically passes through the plane of the galaxy but this is on a period of about 32 million years. It will not happen in 2012, however as it last happened about 3 million years ago.

http://www.americanscientist.org/tem...d/21173/page/2