Great news for Rovio owners... we can finally start hacking code in firmware! This means users can now edit/add the microcontroller code and build custom firmware. This thread is being started to bring in new developers and ideas for Rovio firmware changes.
It started in this thread about eCOS - at the time Wowwee had finally publishing the firmware source code to Rovio. The project was made with ADS 1.2 and CodeWarrior.
The Wowwee Rovio firmware source code is available for download here.
Another member shared a link to a discontinued versions of the ARM Developer Suite v1.2 that was used to compile and link the code. The website is in Chinese.
Chris75 confirmed that the older tools would create a valid target image that could be downloaded into the Rovio flash memory. One version with no significant changes has been posted here by ErFanOnline. Use the Firmware Updater to update your Rovio via USB connection.
This point has been a long time coming, but now there's a lot that can happen that hasn't before.
The full schematic for the Rovio appears to be shown here. There's even a first page that shows how the cables and connections run. Note that the 5.Sensor board has two I2C pins shown on it. That's the place that would be perfect for an additional I2C device to drop in. The new device could reside on the same I2C bus, and intercept and respond to commands sent via the CGI based API!
In yet another post a member gave this link for ADS1.2 downloading.
The old link for schematics is not working, but it shows up on another webpage from the manufacturer here.
There have been a number of users releasing their own custom Rovio Firmware builds. Links are provided in the comment threads.