In the preface to their book (which, it might be noted as an aside, is dedicated to their colleague and friend Samuel Huntington), Mearsheimer and Walt make a point that I think has been overlooked in the larger debate about their argument:

America’s response to that war [Lebanon, 2006] proved to be a further illustration of the lobby’s power, as well as its harmful influence on U.S. and Israeli interests.

That would seem to agree with what I said here, though I will need to read on more to see what they say specifically about Lebanon.