Just wanted to add to this post. I didn't like the response I got, so I set out to find a better solution.
Thanks to a collaboration between Dirk and myself, there's no need to use conditional statements to accomplish this.
Download and install this plugin
Once it's installed you can specify a style for each individual page that will over-ride the style in the stylesheet. Let's say your template has div containing a background image you want to change on a specific page. Let's say it's called "#header".
Go to the page you want to edit in the wp admin and in the "custom tag" field, enter something like this:
<style type="text/css" media="Screen">#header { background-image: url(my_image.jpg); }</style>
That'll do it.