I'm trying to find out exactly when and why the decision was made in
the U.S. for consumer entertainment products (books, CDs, DVDs) to be
released on Tuesdays. I believe this is referred to in the retail
industry as the product's "laydown" date. With certain notable
exceptions (Harry Potter books being one), nearly every title in these
categories is set to go on sale on Tuesdays in the U.S. I believe this
practice dates back to the late 1980s and began in the music industry
with
record release dates. Most recently, it has been adopted by Apple's
iTunes Music Service as "New Music Tuesday." I'm looking for any
details and/or references to the thinking, reasoning and and success
stories behind this practice and if
it is indeed as widespread as I have stated.

This may be of interest:

http://answers.google.com/answers/main?cmd=threadview&id=90606

I did quite a bit of searching on this topic. I thought it was a very
cool question, one that I have asked myself many times. While I never
came up with any direct quotes from record or movie distributors, I
think I have a good idea of why Tuesday is the day. Which is why I'm
not posting this as a comment as opposed to an answer, but if you feel
it satisfies your question, please let me know and I can post it as an
official answer.

The answer to the question posted in the comment above is only
partially true.  Billboard doesn't have that much power in the
industry that it is the reasoning behind Tuesday releases. After all,
Billboard doesn't track sales, or even create the charts. It obtains
all of its information from Nielsen.

You've probably heard of the Nielsen ratings system. Most people
associate the company with TV ratings (http://www.nielsenmedia.com/). 
Although that is only part of what they keep track of. They are also
responsible for ranking books (http://www.bookscan.com/), music
(http://www.soundscan.com/), DVD and VHS sales
(http://www.videoscan.com/), as well as Internet traffic
(http://www.netratings.com/).

According to the SoundScan website
(http://www.soundscan.com/about.html), sales data is compiled and
released every Wednesday.  I would assume that this holds true for all
of the Nielsen divisions.

Therefore, anything released on Tuesday takes advantage of a whole
week of sales, to better their ranking on the charts the following
Wednesday. Higher rankings equates to more sales in the coming weeks,
because these charts are printed in many publications.

"Nielsen VideoScan's VHS and DVD sales charts are currently published
in Billboard, The Hollywood Reporter, Video Store Magazine, The Los
Angeles Times, The New York Times and others."
http://www.videoscan.com/about.html

There is also the aspect of distributing the product to retailers.
Having a specific day to begin sales levels the playing field. (i.e.,
people would favor a store that gets the product out earlier than
anyone else) It also helps prevent pirated copies of CD's and DVD's
from being released before the actual product is released, as stores
are not supposed to even open the boxes ahead of time.

You can read more in these Google Groups threads:
http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&oe=UTF-8&threadm=3a6b5de5.642454%40news.powersurfr.com&rnum=6&prev=/groups%3Fq%3D%2522why%2Btuesday%2522%26hl%3Den%26lr%3D%26ie%3DUTF-8%26oe%3DUTF-8%26selm%3D3a6b5de5.642454%2540news.powersurfr.com%26rnum%3D6
http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&oe=UTF-8&threadm=19990904164035.21765.00003963%40ng-cf1.aol.com&rnum=1&prev=/groups%3Fhl%3Den%26lr%3D%26ie%3DUTF-8%26oe%3DUTF-8%26selm%3D19990904164035.21765.00003963%2540ng-cf1.aol.com%26prev%3D/groups%253Fq%253D%252522why%252Btuesday%252522%2526hl%253Den%2526lr%253D%2526ie%253DUTF-8%2526oe%253DUTF-8%2526selm%253D19990904164035.21765.00003963%252
40ng-cf1.aol.com%2526rnum%253D3

If those insanely long links don't work, try these:

http://snurl.com/247q
http://snurl.com/247r

The first paragraph should read "Which is why I'm posting this as a
comment as opposed to an answer". I accidentally stuck a "not" in
there. If only spellcheck could read my mind.