In the United States, Basel II is expected to apply to only a handful of large, complex organizations, which is the principal reason why the U.S. agencies are proposing only the advanced approaches (A-IRB for credit risk and AMA for operational risk).
Perhaps the main difference between the implementation of Basel II in the United States and the implementation in most other countries is that the U.S. banking agencies plan to retain a revised form of the existing Basel I-based capital rules for the vast majority of U.S. banks; most other countries are replacing Basel I entirely and will apply Basel II to the entire banking system.