I was talking to a customer recently, Greg Duncan, and one of the things that he came up was an idea for a utility to automatically turn the speech recognition engine's state off after a certain amount of idle "speech" time.
A couple emails later, this morning, Greg had already posted the full source code and a link to an installer for SpeechSaver on his blog. I downloaded it, installed it, and checked it out. It does exactly what it's supposed to.
Nice job, Greg.
If you want a solution that automatically turns the microphone off after a set period of time of inactivity, you might want to check it out.