Pretty much this. It would mean I'd be locked into buying books from iBooks.
Not entirely. Only for books that require DRM. iBooks will happily display any non-DRM'd epub-format file. Kobo will too. Kindle won't, unless you run it through a file converter first. Also, if what you want to read is more complex than a novel, the Kindle won't work as well. (This is a drawback of e-ink devices generally--less good handling of graphics and complex formatting. Maybe someday there will be good color e-ink.).