Well now, before posting I visited another page on Apple's site that said just the opposite - an ISBN is required.
But the question is really mute, because even if an ISBN is merely RECOMMENDED, I'm going to get one. So my original question still stands: If I get an ISBN for a book I'm selling via iBooks, can I slap the same ISBN on the Kindle version of the same book.
In other news, it appears that there may be some ebook sellers
that DO require ISBN's.