The American Bar Association issued a report yesterday on the use of "signing statements" by Bush (in particular) and future Presidents (in general). They note that the practice is "contrary to the rule of law and our constitutional system of separation of powers." Some examples,
- Whenever Congress directs the president to furnish information, Mr. Bush reserves the right to withhold it.
- When Congress imposes mandates and requirements on the executive branch, the president often says he will read them as advisory or "precatory."
- When Congress tries to define foreign policy ... Mr. Bush objects. Even if he agrees with the policy, he asserts that the Congressional directives "impermissibly interfere with the president's constitutional authority" to conduct foreign affairs.
- Whenever Congress prescribes qualifications for presidential appointees, Mr. Bush complains that this is an intrusion on his power, even if Congress merely requires that the appointee know about the field for which he will be responsible.(source)
You can read the ABA report here