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Won't happen.

They need to differenciate the iPhone from the iPad in order to maximise sales. iBooks will only be only the iPad in order to create this market segregation.
 
I disagree. They need to get people buying books in iTunes... holdouts may download iBooks on their iPhone, download a dozen books and then decide, "Well, I've already spent $150 on books... so if I'm going to get an e-reader it might as well be an iPad"
 
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