Chaps,
When the engines are at the back they require a large amount of extra structure there to support them. This incurs a massive weight penalty and causes balance problems. It is also difficult to attach a new, decent High By Pass Ratio engine to the back of an airliner. Having them at the back also causes servicing/access problems.
The bending moment relief thing is right. The disadvantages are that the engines suck up more FOD under the wing and they cause problems with assymetry in the single engine case. There is also an issue with the design against catastrophic disc burst of the engine, it being right by the main fuel tank and spar (and passengers).
The overlap between the position of the engines seems to take place around the 80/100 seater mark, with most new aircraft upwards of that favouring the underwing design.
MC