I was working on a landing page for a site a while back, and figured it would be nice to add a slight vignette to the page.
To achieve that I created a 5x5 image, and relied on the browser's image re-sampling engine to make it look smooth.
The end result (excluding IE6 and 7, although you can fix IE7 with a bit of CSS) is a smooth whole page vignette that's bandwidth-cheap and works at any resolution.
Example: /static_media/vignette.html