Here's the last minute modification on the badge PCB: now it has a place for accelerometer ADXL345 or gyroscope/accelerometer GY-521 module. This is a simple demo:

A lot of Arduino guys know those small modules, they are easy to find and relatively inexpensive (about 3 € here). Here's the screenshot of the final PCB version. The place for accelerometer can also serve as the prototyping/hacking area.

I was experimenting a little with those two accelerometers. First I made the service subroutine, which emulates I2C port by software, and now it can even autodetect the accelerometer type. But my opinion is that GY-521 works much better. I have three ADXL345 modules, and all of them have such a strong noise that 3 or 4 low bits can't be used for anything useful. So, my advice is GY-521. It is about 0.5€ more expensive, but it is not only more stable, but also has gyroscope and temperature sensor.