The acid test will be the day some major best-selling author does a book, and they announce it will only be available on Kindle. That will be the first warning that hardball is about to commence.
How the public reacts and (more importantly) buys will determine future publishing directions.
Just my two anyway. 
-40hz
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However, if I felt the Nook was going to get the last of a popular book, the completionist in me would prefer the Nook over any exclusive titles the Kindle may offer because I feel I must have at least invested in that series of books and willing to support the author to really get that final book barring things like Twilights and Harry Potters. On the other hand, if something was exclusive to the Kindle alone, I get the perception that the author must not be putting as much effort in that book and if it did eventually become popular - it will have a non-Kindle version.
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-Paul Keith
It's worth noting that the other outlets now have that book- it just took a few months. I don't know what was behind that shift, as it wasn't as well publicized as the exclusivity.
(And on an unrelated note, I saw the series first, and absolutely loved it, so I can say that while the books are undisputedly better - I think most books are- that's not to say that the TV show wasn't good, just different.)