Is Bush violating the constitutional separation of powers?

The ABA is accusing President Bush of violating the constituion by "his penchant for writing exceptions to laws he has just signed [into law]", and Senator Arlen Specter, Chairman of the Senate Judiciary Committee, has threatened to take Bush to court over the matter.
Read the ABA story.
Here is an excerpt:
The ABA group, which includes a one-time FBI director and former federal appeals court judge, said the president has overstepped his authority in attaching challenges to hundreds of new laws. The attachments, known as bill-signing statements, say Bush reserves a right to revise, interpret or disregard measures on national security and constitutional grounds.
"This report raises serious concerns crucial to the survival of our democracy," said the ABA's president, Michael Greco. "If left unchecked, the president's practice does grave harm to the separation of powers doctrine, and the system of checks and balances that have sustained our democracy for more than two centuries."

It seems to me that it is NOT what the President attaches to the bills before they become law that matter. Rather, it is what he does AFTERWARDS in regards to the questioned attachments that matter. As long as he fulfills his constitutional duties to "take care that the "unvetoed" laws are faithfully executed", then one has NO reason to question his bill-statements now, does one? If, however, he chooses to NOT veto bills but instead signs them into law, but refuses to enforce them, then he's got SERIOUS constitutional problems. The dereliction(s) of constitutional presidential duty to "faithfully execute the laws" may become an impeachable offense.