With the war games in the Gulf, it's time to dust off the War Powers Resolution

It is pretty clear to me that an attack on Iran without Congressional approval would violate the War Powers Resoution and the Constitution:
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SEC. 2. (a) It is the purpose of this joint resolution to fulfill the intent of the framers of the Constitution of the United States and insure that the collective judgement of both the Congress and the President will apply to the introduction of United States Armed Forces into hostilities, or into situations where imminent involvement in hostilities is clearly indicate by the circumstances, and to the continued use of such forces in hostilities or in such situations.

SEC. 2. (b) Under article I, section 8, of the Constitution, it is specifically provided that the Congress shall have the power to make all laws necessary and proper for carrying into execution, not only its own powers but also all other powers vested by the Constitution in the Government of the United States, or in any department or officer thereof.

SEC. 2. (c) The constitutional powers of the President as Commander-in-Chief to introduce United States Armed Forces into hostilities, or into situations where imminent involvement in hostilities is clearly indicated by the circumstances, are exercised only pursuant to (1) a declaration of war, (2) specific statutory authorization, or (3) a national emergency created by attack upon the United States, its territories or possessions, or its armed forces.

CONSULTATION

SEC. 3. The President in every possible instance shall consult with Congress before introducing United States Armed Forces into hostilities or into situation where imminent involvement in hostilities is clearly indicated by the circumstances, and after every such introduction shall consult regularly with the Congress until United States Armed Forces are no longer engaged in hostilities or have been removed from such situations.

http://www.thecre.com/fedlaw/legal22/warpow.htm

Here's an article on the subject of the Supreme Court's Hamdan decision upholding the validity of the WPA, and rejecting Yoo's "unitary exceutive" theory that the President has some sort of inherent power to wage war without Congressional authority: http://www.dailykos.com/story/2007/3/5/94653/16003