Does the word duvet refer to the down comforter itself or to the cover
used to cover it? Are both uses acceptable? Is one more correct than
the other?

Hello.

"Duvet" generally refers to the comforter itself. The comforter's
cover is called a "duvet cover."

From the American Heritage Dictionary:
duvet
NOUN:  A quilt, usually with a washable cover, that may be used in
place of a bedspread and top sheet.
The American Heritage Dictionary of the English Language: Fourth
Edition.  2000.
Hosted by bartleby.com:
http://www.bartleby.com/61/92/D0439200.html

"What is a duvet?
Duvet is simply the French word for down, but it is commonly used to
refer to a comforter. A duvet cover is a comforter cover that protects
your comforter and can be removed for regular washing."
Source: Dewoolfsondown.com Frequently Asked Questions
http://www.dewoolfsondown.com/faq/faq004.php3

"What is a duvet? A duvet is a European bedding component that is
essentially a sewn cotton bag of goose down. Factors to consider in
the quality, warmth and price of a duvet are: the quality of the down
in the duvet, the amount of down in the duvet, and the quality and
type of construction in the shell containing the down."
http://www.eurolinens.com/down/

"The duvet is the comforter itself.  The 'duvet cover' is the cover,
usually
made of two sheets which either button, zip, or velcro together to
enclose  the
duvet."
Newsgroup message, hosted by Google Groups:
http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&client=googlet&selm=19990329121324.29640.00001482%40ng-fx1.aol.com

"Having worked in retail in the past and linens in general, a duvet is
the
down comforter, the duvet cover is the thing that goes over it.  I
have
seen the terms used interchageable for years.  Hope this helps..."
Newsgroup message, hosted by Google Groups:
http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&client=googlet&selm=MPG.116df331a2728782989697%40news-s01.ny.us.ibm.net

"No, that is a duvet cover. A duvet is the down comforter itself."
Newsgroup message, hosted by Google Groups:
http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&client=googlet&selm=7dp6u0%24a2fi%241%40newssvr03-int.news.prodigy.com

As indicated in those newsgroup threads, some people do refer to a
"duvet cover" simply as a "duvet." However, that usage would appear to
be non-standard.

search strategy: 
"what is a duvet"
duvet, cover, "comforter itself"

I hope this helps.