In contrast to broadcast-type media transmission (RealAudio, for instance), latency in a two-way phone conversation can be very undesirable. 150ms is the specified maximum desired one-way latency to achieve high-quality voice. Voice users will notice round-trip delays that exceed 250ms. More than that, and callers start talking over each other. At anything beyond 500ms phone calls are no longer practical. It would be like asking a question and getting the answer after you've gone to lunch. As a reference, the typical delay when speaking through a geo-stationary satellite is 150-500ms.
Delay in data networks is not as critical. An additional delay of 200ms on an e-mail or web page will generally not be noticed, but voice callers on the same network would be hampered by such a delay.