|MadSci Network: Earth Sciences|
The majority of the Earth's land masses being in the northern hemisphere
is nothing more than a coincidence. At various times in the past, much of
the Earth's land masses were clustered in the southern hemisphere in a
supercontinent called Gondwanaland.
As crustal plates drift around the surface of the Earth by the process of plate tectonics you will end up with all types of distributions of continents. The present configuration is simply due to where spreading centers and subduction zones just happen to be located (as you've mentioned, spreading ridges all around Antarctica will act to shove everything northward).
Try the links in the MadSci Library for more information on Earth Sciences.