If the hardware looks OK, then only thing I can think to do is try another program that is meant to work with the sensor and see what it does.
There is a software library for Arduino called NewPing. Search on this site for it and then add it to your Arduino IDE = the instructions are here: http://arduino.cc/en/Guide/Libraries
Close down Arduino IDE and start it again. Look under File - Samples and search for NewPing example programs. I think there must be a simple one that writes to the serial monitor.
If you do try this, check in the new sample program to see which pins it uses to connect to the sensor - it may not be the same as your current program.
Good luck!
Ray