ACM articles on haptics
Since we had a complete physical model for the state of the car that a user was "driving", I could apply pressure back through the steering wheel when the model calculated that the wheel would be hard to turn.
Also, when I was in a 3 day Russell Racing class at Laguna Seca Raceway, I noticed that if the Formula Mazda car that I was driving would get a tire off the main pavement (onto the warning bumps along the track), I could feel this in the wheel of the real racing car. It was not too difficult to determine when a VR user was slightly off the road and to produce similar vibration in the steering wheel of the racing pod - an awesome effect!
Years later, I was the lead programmer on a VR demo system for Disney Quest. We had incredible real time graphics and a good motion platform, but in the demo we did not have haptic force feedback on the oars users would use to maneuver a "raft" down a river in a dinosaur rich environment (I did the AI control for the dinosaurs). This whole VR world was absolutely awesome but the lack of haptics took a lot away from the experience.
I am really busy work-wise, but later this year I am hoping to get some hacking time in on the Open Croquet project that uses Squeak Smalltalk and OpenGL to build and use 3D virtual colaborative workspaces.