Just wanted to put my 2 cents in:
 
A comma should be used after all items in a series (except the last).
     "I bought apples, oranges, and pears today.
Words, phrases, or clauses that "match in grammatical form" and
"importance" should be parallel.
 
Agreeing with me are Simon & Schuster (Handbook for Writers), Shrunk and
White (The elements of style), Prentice Hall (Handbook for Writers), APA
(Publication Manual), MLA ( Handbook for Writers of Research Papers),
Lunsford & Conners (The St. Martin's Handbook), and Lannon (Technical
Writing).
 
Disagreeing with me are (at least two) newspaper stylebooks, The Washington
Post and Associated Press.  Even though I turn to these stylebooks
frequently, I stick with tradition for things like punctuation, as some of
journalism style is based on space (I have no citation for this, but I do
know that The Washington Post, at one point, dropped the second "e" from
employee for this reason, but added it again after persistent complaints).
How often do you see a comma following an introductory (adverbial) phrase
when reading the morning paper?
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Portia Williams
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Division of English as an International Language
University of Illinois at Urbana-Champaign