Jakob Nielsen has two recent posts on usability studies on vertical and horizontal placement on Web pages.  If there was ever a reason to take a hard look at how you've placed information on your site  - here it is.

According to Nielsen:

"Web users spend 80% of their time looking at information above the page fold. Although users do scroll, they allocate only 20% of their attention below the fold."

And with regard to horizontal placement:

"Simply put: Stick to the conventional layout, because it works perfectly with how people look at Web pages:

• Keep navigation all the way to the left. This is where people look to find a list of current options.
• Keep the main content a bit further in from the left.
• The most important stuff should be showcased between one-third and halfway across the page. This is where users focus their attention the most.
• Keep secondary content to the right. It won't be seen as much here, but that's okay — not everything can get top billing, and you need a place to put less-important material."