I had originally planned to allow fine-grained control over the vibration signal. For example, tapping and releasing 0.1 seconds later would send a 0.1 second vibration, and likewise for a long 5-second vibration. However, it turns out Apple only officially supports a single pulse of 0.4 seconds with a 0.1 second rest, making that level of control impossible without using private routines (which would preclude candidacy for the App Store). Furthermore, they have a history of rejecting apps employing gratuitous use of the vibration feature. Finally, I discovered that vibration is limited only to iPhones; even the iPod Touch 4G does not support it, increasing the importance of including visual and audio cues.