ISBNs for eBooks?

I've spent some time recently looking into the question of ISBNs for eBooks. Amazon does not require them for Kindle editions. But Google Editions intends to require them, and has worked out an agreement with the U.S. ISBN Agency (i.e., the Bowker monopoly governing such things in the US) to see to it that this happens. Publishers as a sector are still dithering back and forth as to whether or not ISBNs are truly essential for digital publications. I personally don't see why ISBNs should be integral for products that are not sold via brick-and-mortar retail. Apple, however, wants you to have an ISBN for iBooks. This can turn into an expensive proposition for small presses, what with the traditional fixed-cost of an ISBN regardless of sales. However, per John Chow, Apple has a solution that is worth exploring. Italics are mine:

If you don’t know how to get an ISBN number, Apple suggests that you use an Apple-approved aggregator. These are firms that have a financial arrangement with Apple and can provide you with a number of services.

For example, Bookbaby charge $49 per book the first year and then $19 per book each year the book is in the store. For an additional $19 they will register your ISBN number. They’re convert your eBook to the ePub format for another $19. Other services takes no upfront money but instead take a cut of the profits. You’ll have to decide which service is best for you or you can bypass the aggregators and try to do everything yourself.

Then there is also another question: If ISBN a "must-have," then just one ISBN for all eBook editions of a particular work, or separate ISBNs for Kindle, iBooks, etc.? I'm still digging.