The answer, like many things, is context:
When it comes to designing for users, context is king. A navigation that works well in one context, may not work as well in another. To figure out which navigation is best for your site, it’s important to understand the different contexts where the top and left navigation work best.
Read the rest of the article here. I dig it.
Jason Bradley says
Usability studies from Nielson Norman group show that top navigation is better because most people read from left to right. Top navigation should be used for primary navigation and left can be used as secondary navigation.