Most websites I've built over the years have either had the main navigation placed along the left side of the screen or straight across the top. On the rare occasion, I implement a right-hand design.
While it's true that left-hand navigation is more traditional, common and expected, none of the formal testing that I've done or research that I've read indicates that it is actually more usable.
The only reason I stick with left-hand is because I'm personally comfortable with the placement. I know it works and nobody will be offended by the location. I perceive it as being just one less stumbling block to a person understanding the system I'm designing, but is that a good enough reason to continually stick with left-hand navigation?
In western culture, people read left to right. Considering this, content should logically be placed on the left with the navigation placed on at the right.
During usability testing sessions, I've observed that people will always look at the content first (page titles, headings and graphics) when they encounter a new page to help them decide if they've arrived at the correct location. Placing the navigation on the left side of the page actually forces them to skip over it before they get to the content.
Simply because left-hand navigation has stumbled into popularity doesn't make it the correct choice in every circumstance. Next time you face this design choice, step back and perhaps consider the wiser alternative.