Kate - It is the stores, not the reader devices, that require the ISBNs. I have created two epub books myself; they read very well without those big long numbers anywhere in them.
I suspect you will find that all the big bookstores will require ISBNs; it is probably the unique primary key in their databases. Perhaps Smashwords will sell your books themselves, but won't be able put them to Sony, Kobo, et al without the ISBN.
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