How does "Angry birds backup" app work?
According to the Android Dev Guide, files written to the internal storage such as the Angry Birds data cannot be accessed by other applications by default.  I'm guessing that Rovio used MODE_WORLD_READABLE instead of the default, however.  Maybe this was a mistake or maybe they plan on having future apps being able to read your progress and unlock sometihng, or whatever.

Of course, this doesn't bar the possibility that an Android exploit is being used to bypass the file privacy, but who really knows besides the author of the backup app?