In case you missed Giz Explains the other day—which lays out the entire ebook format and DRM landscape—the iPad will support DRM-free ePub books, in case you've got some on your Sony Reader or B&N Nook. If you've got a Kindle on the other hand, you're SOL, since it uses its very own ebook format. [Apple]
Don't forget the ubiquitous PDF format Matt! Also, you can read the Kindle books on their iPhone/Touch app, so no worries there either.
The real question, and perhaps you guys can call Amazon or Apple to find out, is will the books from these different sellers be different? That is, is there a benefit to ordering the same book from Apple rather than Amazon? Are they the same copies from the publishers or will one be a better version? These are the important questions that would help consumers make better informed decisions. I, for example, would be willing to pay a bit more if one version has better photos or more interactive items which the Amazon copies won’t have at this point. Also, if you already bought a copy of a Amazon book earlier and now there’s one with photos or drawing that were omitted from the earlier version, can we get the newer copy for free or with small upgrade charge??
These are the things informed consumers will want to know. Inquiring minds want to know!