You'd probably need to utalize javascript to accomplish this. As I don't believe there is a stretch option in either HTML, XHTML, or CSS. I could be wrong, but I'm pretty sure there isn't.
Thus, it's usually a good idea anyways to pick a background that can be repeated on an axis or tiled. I have a background that fades into the color black. It always fits the window simply because it repeats in the vertical direction (y). (
my site - warning this is not W3 compliant... This code was written back before I was a bit more strict on that.)