Elsewhere Norman Finkelstein suggests that The Lobby: It's Not Either/Or. Worth a look.
UPDATE: Alexander Cockburn says yes, of course, there is an Israel lobby and although he finds the Mearsheimer and Walt paper to be"extremely dull … [it] has the merit of stating rather blandly some home truths which are somehow still regarded as too dangerous to state publicly in respectable circles in the United States."
Cockburn continues:"Meanwhile, mostly on the left, there has been an altogether different debate, over the actual weight of the Lobby. Here the best known of the debaters is Noam Chomsky, who has reiterated a position he has held for many years, to the general effect that US foreign policy has always hewed to the national self interest, and that the Lobby's power is greatly overestimated."
Cockburn concludes thus:"I have to say I'm not 100 per cent on board with [Noam Chomsky] on this one. The Lobby really does have very hefty clout. Ask Gerald Ford, Jimmy Carter, Ronald Reagan and Bush Sr. In her excellent book The One-State Solution Virginia Tilley makes a persuasive case that the US strategy and tactics in Iraq have more to do with what Israel wants than any self-interested 'realist' US plan."