As far as the cause of the wheel departing the aircraft, I've seen this on the 737 twice (except on the main tire). The 737 and the 757 main wheel bearings are indistinguishable... at least to the naked eye that is. So both of these times a 757 bearing was installed on a 737 wheel assy at the tire shop and the wheel departed the aircraft on take off without the crew knowing anything about it, it was only after taxiing in at the gate that the mechanic noticed a main wheel is missing... one of these two times the mechanic was me. I've also seen aircraft taxiing into the gate with one deflated nose tire... in both of these examples the cause was "not following prodedures" at the tire shop as it is impossible for the mechanic installing the tire to see the problem unless he/she forgets to install an external bearing which has happened before.