The Noise Shirt is a T-Shirt that houses a built in noise level indicator. Wear it in the pub or in the coffee room at work and when you see the indicator hit the red you can either leave, tell everyone to shut up or just go deaf slowly!
I think it would be more useful to have a noise meter built into a cellphone, something one is likely to carry around most of the time. Another advantage is that the cellphone already contains a microphone that could be used for that purpose. One could set the phone to vibrate when the sound level reaches a pre-set threshold. Then one either leaves or goes deaf. Hm, maybe someone could create a software app for the Palm Treo to achieve this?