The backlight on the 3GS was dead, and we have to make it to November before we are eligible for the phone upgrade, so we ordered the display assembly and followed the instructions.
Mostly just like the repair guide shows. One issue and recommendation.
The #3 tab is very small and difficult to discern. There is a black band across the top of the cable, that is NOT the piece you are supposed to lift 90 degrees. In front of that bridle is the tab that you GENTLY lift to release the cable. Ours was white.
For older eyes, use your digital camera in close up or macro mode and take a picture of the area before you try to release the #3 cable. Zoom in on the picture and you can see the mechanism much more clearly.
Inserting the tab on reassembly takes some dexterity, but eventually the cable will slip into the slot, and then just be sure the cable is fully pushed forward before pushing the locking tab back down.