"A Writer's Reference" Fifth Ed., (2003 MLA Update)

Quote from page 237:

USE A COMMA BETWEEN ALL ITEMS IN A SERIES.

Unless you are writing for a publication that follows another convention, separate all items in a series--including the last two--with commas.

Ex: Bubbles of air, leaves, ferns, bits of wood, and insects are often found trapped in amber.

Although some publications omit the comma between the last two items, be aware that its omission can result in ambiguity or misreading.

Ex: My uncle willed me all of his property, houses, and warehouses.

Did the uncle will his property AND HOUSES AND WAREHOUSES--or simply his property, consisting of houses and warehouses? If the first meaning is intended, a comma is necessary to prevent ambiguity.

Ex 2: The activities include a search for lost treasure, dubious financial dealings, much discussion of ancient heresies, and midnight orgies.

Without the comma, the people seem to be discussing orgies, not participating in them. The comma makes it clear that MIDNIGHT ORGIES is a separate item in the series.

AND FOLKS, IT GETS EVEN TRICKIER:

Use a comma between coordinate adjectives not joined by AND. Do not use a comma between cumulative adjectives.

TO LEARN WHY THE RULES ON COMMA USAGE IN A SERIES HAVE SPARKED DEBATES, GO TO:

www.dianahacker.com/writersref

~~ Lolly

P.S. The MLA Convention is the second most used convention in publication writing, the first being the APA (American Psychological Association).