From Nick Gravelyn: http://forums.xna.com/forums/p/49684/298915.aspx#298915
Accelerometer isn't in the XNA Framework anymore. You can access it through these steps:
1) Add a reference to Microsoft.Devices.Sensors.dll
2) Add 'using Microsoft.Devices.Sensors;' to your using statements.
3) Hook up an event and start reading the accelerometer:
try
{
AccelerometerSensor.Default.ReadingChanged += Default_ReadingChanged;
AccelerometerSensor.Default.Start();
}
catch (AccelerometerStartFailedException)
{
}
4) Add the event handler itself:
void Default_ReadingChanged(object sender, AccelerometerReadingAsyncEventArgs e)
{
}
And you're good to go. Keep in mind, though, that accelerometer doesn't work with the emulator so there's no way to really test this without a device. You do need that try/catch because Start will throw an exception in the emulator because it doesn't support accelerometer.