Flip2Mute for Windows Mobile has been released which will allow you to silence your Windows Phone when it is face down.
The application, designed by x86shadow, is similar to the native features on various HTC Windows Phones such as the HD2. Your Windows Phone needs to be running Windows Mobile 6.5.xx, have .NET CF 3.5 installed, an accelerometer, and be a WVGA device.
Along with silencing your phone, you can set Flip2Mute to turn off the screen or set the phone to vibrate. This works out great for those who are constantly in and out of meetings and need a simple way to mute their Windows Phone. Simply turn on Flip2Mute and set your Windows Phone face down on the conference table. The only bug I experienced in testing Flip2Mute was that occasionally the phone stayed on silent even after being turned face up.
Flip2Mute is a free application and if your interested in giving it a try, you can download it here.