On the skinny
My monitor has a resolution of 1280x800. Most websites nowadays are designed with a minimum resolution of 1024x768 in mind. I believe this is rather an aggressive resolution requirement. Even though my monitor's resolution is greater than this requirement, I have trouble viewing many web pages. This is because I have a tablet PC, and it can be switched to portrait mode. Even though portrait mode _feels_ like it should be better for web browsing (indeed, it is quite excellent at reading PDFs), it turns out that web pages always leave nasty horizontal scrollbars.
While I don't know anyone with a resolution on their monitor of less than 1024x768, I know quite a few people with a resolution of 1024x768 exactly. Newer devices like the Eee PC, the iPhone, or the Nokia n800 or even my tablet, have resolution widths of 800, or perhaps even slightly less. These devices are built to read web pages proper instead of some cut down web. In addition, the devices have a substantial resolution.
Having horizontal scrollbars is unacceptable in today's environment. Even CSS has become fairly modern, so there should be no excuse to have a website that is not flexible enough to work in all environments, especially given the variety of devices which are now being used to view these pages.