Portland Police Bureau Tracker
There are two phases to this idea - an initial one that can be done now, and a much more complex one that would take some advanced processing and decent hardware. In the first phase, you take the 911 data and stream it out through twitter so people can easily see all the 911 calls. In the second phase, you use the live police scanner audio stream provided by the Oregonian, or roll your own, and have it automatically transcribed using voice recognition software. Over a very short period of time, you could train the software to recognize individual voices, and tag messages with ids of people speaking. You then push out short transcripts of the captured audio automatically.