It is a commonplace element in our daily aural landscape. It's recognizable enough to be included in modern movies and TV shows, almost completely replacing the electronic "chirp" as the notification of choice on network police procedurals and dramas. You all know it, but in case you can't quite bring it up, here it is:
What we don't always stop to consider, however, is that sound was designed. Like so many other bits of audio input in our lives, there was care and consideration put into the making of that little three-note riff.
Kelly Jacklin, designer of the "tri-tone" sound, goes into some detail on the thought process and the mechanics of creation surrounding this little iconic bit of audio. I find it fascinating how she handles the permutation of options (MATHMATICALLY!) and then auditioned each one. She includes a short track of rejects in her post as well.
One final detail of note is that her sound was not designed for the iPhone at all, but for a piece of software written by a friend, that was bought by Apple and eventually turned into iTunes, which eventually migrated the sound onto the original iPhone. Interesting and inspiring how people's creations can take on a life of their own, far beyond the original intent of the creator.