Wow, this has an utterly horrible interface. The way it bobs up and down as a I scroll is quite appalling. I'd rather have frames than this.
It's absolute-positioned, and they're updating its 'top' when you scroll. WebKit has a fast enough JS engine that this sort of thing is seamless, but not every browser does / not on every computer.
edit: tried Firefox (4 beta), it's a handful of pixels off the top, and bobs. You'd think people would know about position: fixed by now.