To add to Mathew Hoy's technique of a Liquid, Rounded, 4-Corner Masthead -- an adaption to Dan Cederholm's original 4-Corner Rounded Technique -- I've gone ahead and created a Liquid, Rounded, 4-Corner Content Box (LR4CB), a.k.a. Mouthful.
Coincidentally, while the masthead is not a fully-fluid (as in both horizontal and vertical re-sizing), it does in fact grow horizontally based on your browser window's width. Go ahead and re-size your browser window, and you'll see the masthead grow horizontally in size.
You'll also notice the masthead makes use of drop-shadows for the rounded layout. The technique is fairly simple and was created in Photoshop. The original image was sliced to accurately include the faint shadow that fades into the page background.