The cry detector running on the iPAQ under Linux is an implementation of the baby monitor, one of the applications announced in the Impromptu project. Unlike the other Impromptu applications, the baby monitor doesn't need the bandwidth for sound transmission all the time. It opens a bi-directional audio channel only when the baby starts crying.
One can think of detecting different types of interesting sound patterns. In many cases, the detection can be made computationally cheap.
Simple sound pattern detectors of this kind are highly scalable and can be used
in a variety of applications.