Firstly I must point out that I am in no way advocating horizontal scrolling on web pages.
Assuming that you are using a screen resolution of at least 1024×768 pixels (96% of internet users worldwide in 2009), you will almost never see horizontal scrolling. This is because it has been recognised as highly unusable for some time. I wont be debating that here.
However, I take issue with the fact that vertical scrolling seems to have been tarred with the same brush.
Its extremely common for a client to request that users should not have to scroll to see any content on their pages (or a large majority of the content at least). This seems to be based on two assumptions. First is that users dont see content below the page fold (the point at which the content disappears behind the frame at the bottom of the web browser). Second is that users dont like scrolling and find it a chore.