I am surpised no one has done this before. Basically the robot goes thru a brute force attack on the phone by using the touch panel and trying all the possible different combinations. The authors will be publishing the robot (can be printed out with a 3D printer) and it can break a phone within 20 hours. Expect more news and data when its gets published/presented at Defcon really soon. Oh it costs around $200 to build.
http://www.ubergizmo.com/2013/07/robot-cracks-phone-security-code-with-brute-force-pin-input/