From my iPhone's perspective, fixed width layout is problematical when using code blocks. The scrollbar for wide code blocks doesn't show up, so I can't read the far right of the block.
Otherwise, I think it's a simple matter of what kind of site you're designing and how it looks on different size screens and windows. As previously mentioned, there's an option to set a maximum width, but the same caveat applies to code blocks and iPhones. I've designed both, and I don't prefer one over the other.
Although, it's fun to watch the boxes move around as I play with the browser size with a fluid layout, but I can be easily amused.