The Globe's Charlie Savage writes about the GAO report on the subject and highlights a good example:

For example, one law requires the Customs and Border Patrol to relocate its illegal immigrant checkpoints near Tucson every seven days to prevent smugglers from being able to predict where they are, but the agency failed to do so. The border patrol told the GAO that the law is flawed because it "diverts resources," and it characterized the requirement as "advisory." In his signing statement of Oct. 18, 2005, Bush instructed the border patrol to view the "relocation provision as advisory rather than mandatory" on the assertion that only the president has the constitutional authority to decide how to deploy law enforcement officers.

Let me first say that I agree with Bush that the provision in question is supremely stupid and should not be the law. But Bush does not have the power to declare it "advisory." He must either veto the bill or have it challenged in court. The Federalist Papers, as always, are instructive. In Federalist 73:

The first thing that offers itself to our observation, is the qualified negative of the President [the veto power] upon the acts or resolutions of the two houses of the legislature; or, in other words, his power of returning all bills with objections, to have the effect of preventing their becoming laws, unless they should afterwards be ratified by two thirds of each of the component members of the legislative body.

The propensity of the legislative department to intrude upon the rights, and to absorb the powers, of the other departments, has been already suggested and repeated; the insufficiency of a mere parchment delineation of the boundaries of each, has also been remarked upon; and the necessity of furnishing each with constitutional arms for its own defense, has been inferred and proved. From these clear and indubitable principles results the propriety of a negative, either absolute or qualified, in the Executive, upon the acts of the legislative branches. Without the one or the other, the former would be absolutely unable to defend himself against the depredations of the latter. He might gradually be stripped of his authorities by successive resolutions, or annihilated by a single vote. And in the one mode or the other, the legislative and executive powers might speedily come to be blended in the same hands.

. . . But the power in question has a further use. It not only serves as a shield to the Executive, but it furnishes an additional security against the enaction of improper laws. . . .

The Constitution provides for the veto power. In Clinton v. New York, the Supreme Court stated that:

In both legal and practical effect, the President has amended two Acts of Congress by repealing a portion of each. “®epeal of statutes, no less than enactment, must conform with Art. I.” INS v. Chadha, 462 U.S. 919, 954 (1983). There is no provision in the Constitution that authorizes the President to enact, to amend, or to repeal statutes. Both Article I and Article II assign responsibilities to the President that directly relate to the lawmaking process, but neither addresses the issue presented by these cases. The President “shall from time to time give to the Congress Information on the State of the Union, and recommend to their Consideration such Measures as he shall judge necessary and expedient … .” Art. II, §3. Thus, he may initiate and influence legislative proposals.27 Moreover, after a bill has passed both Houses of Congress, but “before it become[s] a Law,” it must be presented to the President. If he approves it, “he shall sign it, but if not he shall return it, with his Objections to that House in which it shall have originated, who shall enter the Objections at large on their Journal, and proceed to reconsider it.” Art. I, §7, cl. 2.28 His “return” of a bill, which is usually described as a “veto,” 29 is subject to being overridden by a two-thirds vote in each House. . . . There are powerful reasons for construing constitutional silence on this profoundly important issue as equivalent to an express prohibition. The procedures governing the enactment of statutes set forth in the text of Article I were the product of the great debates and compromises that produced the Constitution itself. Familiar historical materials provide abundant support for the conclusion that the power to enact statutes may only “be exercised in accord with a single, finely wrought and exhaustively considered, procedure.” Chadha, 462 U.S., at 951. Our first President understood the text of the Presentment Clause as requiring that he either “approve all the parts of a Bill, or reject it in toto.” 30 What has emerged in these cases from the President’s exercise of his statutory cancellation powers, however, are truncated versions of two bills that passed both Houses of Congress. They are not the product of the “finely wrought” procedure that the Framers designed.

So much worse would be a process where the President unilaterally usurps legislative power. What President bush has done with signing statements is flagrantly unconstitutional and an affront to the Constitution.