What is the license on mac wallpapers?
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