When can an individual on an H-1B start working?

This question is answered in light of legislation introduced as part of the American Competitiveness Act of the Twenty First Century [AC21]. The portability provisions allow a nonimmigrant alien previously issued an H-1B visa or otherwise accorded H-1B status to begin working for a new H-1B employer as soon as the new employer files an H-1B petition for the alien. Previously, aliens in this situation had to await INS approval before commencing the new H-1B employment.