What I think it comes down to is the experience one looks for. Is the story alone enough or is the story plus the weight of the book and the smell of the pages what you're after? These could be two different kinds of people who will make different buying choices.
I also think that this change will depend largely on how accessible electronic technology will become. There is still the issue of energy and affordability to consider that I think will keep this new age of ebooks at bay for a bit longer. There are still plenty of people who don't quite have the same access to ebooks and would probably prefer the traditional route.
As for my book cover designing process, I do generally have an end format/feel in mind for the book. I've never designed an entire book before but I did consider the overall look and feel for a physical book when I was designing my cover.
Designing for an ebook could be just as involved though. There's a lot of interactivity possible with an ebook that you can't get with a physical book. That sort of interactivity could draw readers in the same way covers do for plenty of folks now. Who knows what that sort of capability could do to the experience of reading? It is faster and more convenient, but it could be a more dynamic experience as well. Physical books won't be going anywhere for quite some time, but there's lots on the horizon for books in the future!
