The best essay written about product placement is by Mark Crispin Miller
in his engaging anthology "Seeing Through the Movies."  I believe Miller
also has some association with the Center for the Study of Commercialization
in Washington, D.C., which has made a number of statements on this issue.
 
By the way, if you want to see a particularly egregious example of product
placement, check out Ron Howard's lame newspaper film "The Paper."  Coke
plays such a prominent role I almost expected to see their name listed
in the credits.
Brian Rose
Fordham University