The link 1Daniel1 posted has the correct information about how to add a pressure sensor (also known as a bumper) to the URDF of a model. However, whenever you try to listen to the contact sensor topic, you'll most likely get a variation on this message:
Error [ContactSensor.cc:237] Contact Sensor[contact_sensor] has no collision[base_link_geom]
This is because the contact sensor implementation is broken in the version of Gazebo that comes with Fuerte. The comment thread on Gazebo bitbucket issue 13 has information about how to get a version of Gazebo with working contact sensors. (I'd link to it, but I don't have enough karma.)