I breezed through a usability study comparing left- and right-justified site navigation this morning.
I had the standard left-justified navigation for a while, and at one point had a three column layout with sidebars on both sides of the page, but in one of my redesigns I decided to go with the current right-justified navigation, and plan on sticking with it.
My basic reasoning is that this layout emphasizes the content over the navigation. As English speakers read from left to right, the content area has dominance. It also (I think) makes it a little easier to track your position on a page when reading a long post. With left-justified navigation, when you reach the end of a line on a page and move your eyes back left, you need to account for whatever space is taken up by the navigation bar. Using a right-justified navigation scheme, you just let your eyes snap all the way to the left of the page, and no searching is necessary.
Now, these are my opinions only, and I’m not schooled in usability at all, so I could be completely off base with that, so take my reasoning with a grain of salt. It’s just my thinking on an admittedly not very important matter. ;)