I was trying to map out in my mind what options I'd have, in terms of sources for buying books, and the implications in terms of consequent limitation on platforms I'm likely to read books on. I came up with this, a bit arbitrary, but:
It shows the eBookstores, and then which of the eReaders can handle (DRM) content puchased at the Bookstore. I did what I could to figure this out correctly, but it's not always easy, so don't take this as gospel, feel free to correct me if I got something wrong. I discounted anything that got too exotic, stripping the DRM, running through conversion software, etc., just basically move the file to the device kinds of things.
One aspect that kind of struck me as ironic, is that the Apple bookstore may be one of the worst places to buy from, cross-compatibility wise, but other than the PC, the iPad was one of the best readers, in terms of support. Kind of ironic, given the Beta-vs-VHS thing going on between Amazon and B&N/Borders/Sony, I guess they missed the fact that by trying to support the Apple iPhone/iPad, and not help the competition, they maybe helped Apple sell eReaders in place of their own offerings. I don't think customers are going to be too hung up loading up different store/readers on an iPad, they'll buy from whomever is cheapest, supports the other platforms they have/want. I know there are a lot of Sony fans out there, but they're kind of weak on the Mobile phones front, seems like it's a weak strategy compared to the others right now. But if you buy an Apple iPad as your eReader, then you can download content from most of the majors, Amazon, B&N, Borders, etc. Interesting.