Question: Okay, what is Amazon doing? They put all their CSS definitions in the `<head>` tag? I can see the purpose of saving requests, but it doesn't seem very dynamic to me.

Any ideas?

Answer: 
The `<head>` element is traditionally the place for CSS files because the browser will download the files at the beginning of the request.
Putting the css on other place then head will trigger a rerendering of the side in some browsers. So if you put your css at the bottom of the page you get an unstyled page until the css is loaded and then the whole page will be rendered again with the new style rules.
<http://developer.yahoo.com/performance/rules.html#css_top>