All I can say is *WOW!!!* this robot is amazing!
This NXT robot firstly sweeps over the sodoku board to find where the existing numbers are. It then scans each existing number individually and does some trick image recognition (segmentation/thinning etc) to work out it's value. Once it knows the values of each of the existing numbers, it can solve the puzzle. Using the pen on the front it proceeds to fill in all the blank space.
It would be interesting to find out how long the whole process takes.
More details at http://tiltedtwister.com/sudokusolver.html