@ptriv The main conclusion is something that is supported by everything else, but it does not support any other point. "This suggests" also provides a clue that it is a conclusion. So the main conclusion is that "recent proposals to allow use of cell phones on airplanes are ill-advised". All of the premises in this argument lend support to that conclusion. The main conclusion in turn does not support any other statements itself.

"Cell-phone use would be far more upsetting on airplanes than it is on buses and trains" is also a conclusion but in this argument it is a sub-conclusion/main premise and not the main conclusion. The last two sentences are premises that support this sub-conclusion but the sub-conclusion itself lends support to the main conclusion. You know that it is not the main conclusion since it also lends support to another point in the text.