The main reason driving the 42V system is the ability to use solenoids to directly actuate the intake and exhaust valves in the engine, eliminating the camshaft, lifters, pushrods, and rocker arms (and all the associated losses/friction). Direct computer control will also allow much more advanced VVT (variable valve timing) systems, reducing emissions. The current required to actuate valves at 12VDC is impractical, requiring huge wire and switching transistors.
Other accessories that will be electrically driven will be power steering pumps and A/C compressors. Running these at a constant speed pays dividends in efficiency and design simplification (cost), but power requirements make this impractical at 12 volts.
AFAIK, the new 42V cars will restrict the high voltage to systems that would have some real benefit. General lighting and accessories will remain at 12V, developed from the 42V supply by an electronic regulator.
[This message has been edited by NJwirenut (edited 01-27-2004).]