http://www.slate.com/articles/techno...re_s_why_.html
The profitability of Kindles and Amazon Prime aside, it doesn't make sense to me. For this deal to make sense, the customers buying it must a) buy enough ebooks for the Kindle to be profitable, or b) buy enough stuff online for Prime to be profitable.
At this point, with the lowest-priced Kindle being $79, and with a price drop likely this year, isn't it likely that most of the people who want one, except those who are in financial difficulty (and thus unlikely to do a) and b)), has bought one already? If you buy so many things from Amazon that Prime sounds like a good deal, and you do want a Kindle, why haven't you gotten one already?
I think Amazon is likely to promote the Kindle in other ways, such as the 40% with Amazon credit card, or lowering the price, or packaging an Amazon gift card with Kindle purchase, etc.