What notice must I provide to the U.S.?
This depends on your citizenship. Most holders of US visitor visas can just board a plane (or bus, or train, or ship) and go to the United States at will, without notifying the government in advance. However, if you are a citizen of P.R. China, and hold a multiple entry B-1 or B-2 visa, you need to enroll into EVUS (Electronic Visa Update System) online: Electronic Visa Update System (EVUS) Frequently Asked Questions The EVUS process is apparently similar to the ESTA process required for traveles on visa waivers. As with ESTA, an EVUS enrollment is valid for 2 years, or until your current passport expires, whichever is sooner. As with many other innovations in the area of visa and passport regulations, it remains to be seen if the EVUS system will be extended to citizens of some other countries as well.