Is Netanyahu's address to Congress unconstitutional?
Likely yes, says University of San Diego's Michael Ramsey, for the reasons enumerated here. Basically, the point is that the president and not Congress has the constitutional authority to receive foreign leaders.
I'd put the question slightly differently. Netanyahu can't be acting unconstitutionally, because he's not bound by the American Constitution. So, the question is whether the House leadership's invitation to Netanyahu to address Congress without going through the president, and then receiving Netanyahu once he accepts the invitation, is unconstitutional.
House Majority leader Boehner indirectly addressed this issue in a comment I heard on the news the other day. He suggested that given that President Obama has been ignoring Congress' constitutional prerogatives, as by unilaterally rewriting immigration law, Congress can retaliate by ignoring the president's constitutional prerogatives.
But two constitutional wrongs don't make a right, and since I give Obama a hard time when he acts unconstitutionally and contrary to the separation of powers, I hereby give Boehner a hard time for inviting Netanyahu despite the absence of any apparent constitutional authority to do so.
UPDATE: A few points arising out of some comments I've received.
(1) Just because no one has standing to sue to stop something, doesn't meant that that something is constitutional. Our elected representatives have an obligation to obey the Constitution even when the courts can't and won't get involved, and, for that matter, even when no one is looking.
(2) Here's the logic of the "unconstitutional" position: (a) the power to receive ambassadors and public ministers a foritiori includes the power to receive foreign heads of state when they are here on diplomatic business; (b) this power is lodged by Constitution in the executive, and, explicitly given to the executive, Congress can't exercise it unless at the invitation of the president. There is good reason for this, because while Congress has a role in foreign policy, direct diplmoatic relations with foreign governments are exclusive in the executive, so the nation speaks with one voice-Congress could not have its own State Department, for example; and (c) Netanyahu is coming to speak to Congress on a formal diplomatic mission, not for mere private conversation or consultation, and thus the president has to give his assent, or at least Congress shouldn't do it over the president's objection. None of these three points is unassailable, but they do strike me as likely being right.
(3) As a commenter pointed out, with regard to Obama and immigration I argued that Obama's actions probably are not technically unconstitutional, but they do violate constitutional norms regarding the respective roles of each branch of government, and thus undermine the separation of powers. Similarly, even if Boehner does have the authority to invite Netanyahu for a diplomatic address, doing so against the president's wishes violates constitutional norms that have been observed for generations. The fact that the Republicans believe "Obama started it" doesn't strike me as a great response.