So he should just sign any and all un-Constitutional legislation that reaches his desk?
To do otherwise would set a horrible precedent. Imagine a future clinton-like president vetoing a narrowly passed anti-porn or pro-school-prayer bill simply because he thought it was unconstitutional. You would (rightfully) be screaming that the president cannot make that judgement, and that the majority is being denied the right to be heard instead by the Supreme Court.
Many a law has been overturned when their initial passage was unquestioned (Roe v Wade, anyone?) or upheld when thought unconstitutional (any gun control law), so it is difficult for anyone to say how the highest court will come down on an issue. Presidential second guessing on the constitutionality of a bill will only lead to no good.
But why does it have to get to the President's desk before the public becomes concerned about the constitutionality of the bill? Let's put pressure on the sponsors and those who will be voting on the contents. Moreover, a court that believes in interpreting rather than rewriting the constitution would be helpful.
Also I'd like to see the a lower court involved in these controversial bills prior to their passage, saying that the contents meet constitutional muster. Why fight for years to pass a bill that will just be overturned (other than posturing)? But I don't see this happening in my lifetime.