Is An Ebook a Book? And What Does It Mean When You Buy It?

I've been weighing in on a fairly interesting post on the Amazon Kindle forum.  The focus is on the idea of getting an ebook version of a physical book when you buy it.  Myself and the original poster think that it'd be a great way for a publisher to add value to their product (I hate using that phrase, though):  a sort of bonus, a thank-you, an incentive for the customer.  Some of the others in the thread feel that that's basically asking for two copies of the book for the price of one...