Yes! The main reason is widescreen monitors, but you still have to support narrower window sizes. Some people still have smaller screens (or even mobile devices if you don't have a mobile site), which can result in the dreaded horizontal scrollbar. It's better to build a fluid width layout to support everything. But it's an order of magnitude harder to build a fluid width site, since you have to test and design for fluidity. Images, for example, aren't naturally fluid. That's why you hardly ever find a fluid-width slideshow or image gallery out there (that might make a good blog... Fluid width galleries and slideshows).

One of the coolest fluid width sites is Wishzilla.com, which makes use of layered transparent PNGs to make the background look great at both the widest and narrowest widths.