The 2nd Edition FAQ has been updated with a rules change regarding additional US income in the case of an unprovoked declaration of war by Japan:
Page 9, The Political Situation, The United States: This paragraph should read “The United States begins the game neutral. It may not declare war on Japan unless Japan first declares war on it or makes an unprovoked declaration of war against the United Kingdom or ANZAC. Following any such unprovoked declaration of war by Japan, the United States will receive an immediate one-time bonus payment of 30 IPCs, representing the total mobilization and transfer of military assets within the continental United States. However, if the United States is still not at war with Japan by the Collect Income phase of its third turn, it may declare war on Japan at the beginning of that phase. This is an exception to the rules for declaring war (see “Declaring War,†page 11), which may normally be done only at the beginning of the Combat Move phase.†(Note that this change applies only to the Pacific rules, and not to the Global rules.)