I am perfectly willing to consider the argument that Flast was wrongly decided and taxpayers qua taxpayers shouldn't have the right to go into court to challenge any expenditure they don't like.

But Dellinger's argument that the reason for this is because the other branches should have a coequal or similar authority to interpret the Constitution as the judiciary is a very bad argument. He is a former Justice Department official, and of course, institutionally, the Justice Department tends to favor its own power to interpret the Constitution. Nonetheless, the problem is that this presents a slippery slope to all sorts of bad consequences, including Executive officials feeling they have the right to disobey judicial interpretations of the Constitution as well as laws of Congress that they don't agree with-- even if the President signed those laws rather than vetoing them!

The Bush Administration has effectively demonstrated these problems to a greater extent than could ever be imagined. Attempts are made to rely on standing and jurisdiction doctrines to keep matters out of court, and then the Executive Branch comes up with completely unreasonable and ridiculous interpretations of the law that are insulated from challenge. Signing statements are used to neuter statutes that are signed into law by the President.

Chief Justice Marshall in Marbury v. Madison had it right. It is entirely the province of the judiciary to say what the law is. Any other approach leads to a potential dictatorship.