Brace yourselves for... an on-topic post.
I disagree with braver's initial stance that purchasing a paper book automatically entitles the owner to a digital copy provided by the publisher. Purchasing media in one format does not entitle you to other formats; e.g. if I buy Halo for the XBox, that does not automatically give me a copy of Halo for PC (unless the publisher chooses to offer it that way). As far as I know, you are legally entitled to scan your paper book for your own purposes, but obviously this is not feasible for an individual.
You should also consider that the overwhelming majority of books were bought long before digital versions were available, let alone considered as a viable option. If you bought a vinyl record in 1975, you may be entitled to convert that record to a digital format -- at your own cost. Are you entitled to an MP3 provided to you by, and therefore at the cost of, the publisher? Should current media purchases include a future "conversion surcharge" or "format upgrade protection" fee? (I'm sure THAT would be a real winner.
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And how, exactly, should the publisher verify that you have the book? You could have sold, lost or damaged it since the time of purchase. Even just co-ordinating a project like this would require significant expenses, since you would have to provide a proof of purchase and ownership, which would need to be verified somehow.
Last but not least, ideally an ebook should (and hopefully soon, will) be all that you need. Bundling electronic and paper versions together should be, at best, a stopgap on the way to the increasing market share of ebooks.