When we see the iPad, and it's awesome books, we're gonna have to have a backup of our books on our macs. After all, if our iPad dies, or gets stolen, breaks, or whatever, it'd suck to have to spend god knows how much $ to rebuild our eBook collection.
So, do you think we'll see iBooks as a Mac/PC app, the start of separating iTunes to reduce bloat, or will it be the big iTunes 10 feature?