You may or may not have seen this blog post from the CSS WG asking for feedback on backgrounds and borders in CSS3. One of the features in the CSS3 draft is the ability to specify multiple backgrounds on a single element, thus allowing for effects like Sliding Doors to be achieved with only one element instead of two. Since people are snooping around in the WebCore code, I might as well reveal this now rather than later.
Safari 1.3 and 2.0 support multiple backgrounds as specified in CSS3.
I encourage web designers to try out the support, play with it, see if the syntax makes sense, and give the working group your feedback. This feature should be considered experimental in Safari, and we will update our implementation as necessary to match the CSS3 draft as it changes.