Hey smart guys, does shifting into neutral with an automatic car...

The voltage is maintained within certain perameters, but the output current (amperage) of the alternator varies depending on the loads placed upon it.
Sometimes, if a heavy load is placed on the alternator, the belt can start squeeling (if not tight enough). This is because loading the alternator makes it harder to turn, more of a load on the engine.

http://www.cartalk.com/content/columns/Archive/2001/May/07.html

http://www.boston.com/cars/news/articles/2008/06/15/drive_it_forever/

http://www.straightdope.com/mailbag/mradiomileage.html

http://www.instructables.com/id/Save-electricity-with-your-gas-guzzler/?comments=all
"An alternator typically takes about 1 HP (horse power) for every 25 amps of power generated. So, a 100 amp alternator will require about 4 HP at full output. Most alternators do not operate at full output for very long."