We started this project for a hackathon event here in Estonia. In 48 hours we built a working prototype from a couple of small servos, a lipo battery and a BL600 BLE module. This thing has come a long way since then and has been in development for almost a year now.
The final version has an Invensense IMU, a Nordic BLE SOC and wireless charging. Everything is built on one pcb with most of the mechanics being 3d printed at the moment (mostly by Shapeways as our own printers can really do the required precision parts). We have learned so much about inertial sensors from this project. The first versions were almost uncontrollable due to the way a differential drive robot inside a sphere works. With just one point (ideally) contacting ground directional control was all but impossible. The latest versions use a highly integrated IMU that does a lot of the integration and filtering actually in the motion controller so our M0 core in the BLE SOC has more time to handle everything else.