We noticed some time ago (completely by accident) that browsers will display more or less of a block's CSS background-image depending on window size, never allowing an image to overflow its containing block. It's a subtle, yet extremely elegant effect, as evidenced by the v-2 Organisation's site. We've also created our own simple example of this technique.
Title Picture accomplishes the same effect (to predictably fine results) on other images (not backgrounds) using the overflow: hidden property. (via Zeldman)