Parallax is a ridiculously awesome technique (If you're not familiar with it, go to http://silverbackapp.com/, grab a corner of the browser window and drag to resize). It's surprisingly easy to implement, to boot. And if you're using Sass for your CSS, it's even easier now--just use this mixin.

The only requirements for using this mixin are:
1) Alpha transparent images for your background, midground, and foreground, and
2) An alternative solution for IE6.