(AZ immigration law that imposes sanctions on employers who hire illegal aliens is not pre-empted by federal law (The Immigration Reform and Control Act) because AZ law does no more than impose licensing conditions on businesses operating within AZ; law not preempted by E-verify because program only voluntary at national level and no congressional intent to bar states from mandating participation; Court stated, contrary to claims by various state attorneys general, that AZ law fell "well within" the confines of the authority the Congress chose to leave to the states; Court's opinion demonstrates immigration is inherently a local and state problem and national law not overall solution due to need to tailor solution to needs of each particular state).