Bush has been breaking US law by authorising unwarranted surveillance on US citizens by the National Security Agency:
the law forbids warrantless surveillance of US citizens, and it provides procedures to be followed in emergencies that do not leave enough time for federal agents to get a warrant. If the NY Times report is correct, the government did not follow these procedures. It therefore acted illegally.
Bush's order is arguably unconstitutional as well: it seems to violate the fourth amendment, and it certainly violates the requirement (Article II, sec. 3) that the President "shall take Care that the Laws be faithfully executed."