I just wanted to point out that any discussion of Apple's soon-to-be-released device should mention that this device will be backlit, just like normal computer displays and iPhones. Many people don't feel comfortable reading for long periods of time on a backlit screen. This is why the e-Ink technology used in the Kindle, Sony and Nook have proven to be so popular, it's just as comfortable reading on it as reading a physical book.
I have the Kindle app on my iPhone and I love it for when I want to read for ten minutes on the subway. But if I'm going to do some serious reading (say, longer than 20 minutes), I go to my Kindle. I would never want to do that kind of reading on a backlit screen.
I have the B&N eReader on my BlackBerry, but I never stay on it for long. I'll pull out my nook if I'm planning to read for a lengthier period of time.