dgatwood said:
Or you could just take a pair of needle-nose pliers and squeeze the metal shield of the connector a little bit so that it will be tighter. But before you do that, try a different USB cable. It might be the cable connector that is stretched rather than the one in the camera.
I would agree with dgatwood. The USB socket in the camera can have the metal bent from front-back stress on the cable. (That's why I recommend a right angle cable so that the cable hangs down) I am not sure about the 5D2, but on the 60D there is a screw where you can remove the faceplate around the connectors. If you can remove the plate you should be able to pinch the bent metal of the usb connector shell back together so that there is a decent amount of friction when you insert the cable. If the plate does not remove easily, you should be able to use a jewellers screwdriver to bend the metal back.