The fonts themselves will likely be copyrighted because Truetype fonts are considered computer software that can be copyrighted. The images will also likely be copyrighted as images, unlike the images of letters of a font. On the other hand you are free to create your own Emoji font with the same themes.
You can go to http://www.apple.com/legal/sla/ which has the license for about any Apple product, and then check the iOS and MacOS X licenses which I believe say something about fonts, voices and so on. Obviously you will have certain rights as a legitimate user of their software (and as a software developer for iOS and MacOS X, your users will have these rights), but you need to read the license to find out what the limits of that license are.