Ros on Raspberry pi
This wiki contains information on installing ROS fuerte on the Raspberry Pi.
There are other ROS versions that you can install. Here is another wiki page that describe the procedure.
Installing ROS on Raspberry Pi:
1: Follow ZRob314's Tutorial and write the image located here onto your 4gb sd card.
2: ROS fuerte will be pre-installed in the image file so you will not have to build anything.
3: (Optional) Using the rpi-clone script clone the contents of your 4gb sd card onto a higher sized sd card to get more space.