As a user (not a developer), what are the license terms governing use of the included wallpapers on broadcast media, for example, a website?
Could I, for example, use the mac wallpaper as a starting point for building a banner to my website?
Here is the Mountain Lion end user license agreement that is your best starting point for what Apple feels it has actually sold you for your money.
Of course, you'll want a lawyer to give you advice on what is customary, but the artwork isn't explicitly called out as something you are prohibited from using commercially. However, you would want to be very sure of trademark infringement issues if a reasonable person would recognize that you are re-using Apple's works for commercial benefit.
I personally would steer clear of using this and pay someone to provide you with your work or grab thing that are shared more explicitly under a creative commons license. Apple does indicate that you can use fonts under some restrictions (make something with them - don't just resell them) and you will see in the application specific licenses that in general you don't get intellectual property rights to any assets included in the software unless it's called out explicitly as done for the loops in GarageBand. The obvious uses like loops for music software are given knowledge base articles, but items like wallpaper don't get an article and you are expected to be able to read the license and perhaps between the lines of what the licenses don't say.
Basically, you probably haven't been granted the right to re-use the assets, so use common sense if you don't know it's OK or have asked for permission to use it. You might find out that Apple has licensed these and you can use them, but I don't see any wording to call this out in the license terms.
Apple sources their wallpaper from others so it's not free use for any commercial projects. If you plan on making money from their source without modifying it severely, you could be sent a takedown request.
The Galaxy wallpaper in Lion is based off the Robert Gendler Andromeda picture: robgendlerastropics.com. They also got into some hot water for not paying the original artist for one of their iPad wallpapers, which was resolved: http://news.softpedia.com/news/Apple-Uses-Unlicensed-Photo-for-iPad-Wallpaper-134605.shtml