I think ebooks are changing fiction in interesting ways. Print book length today is decided mostly by marketing forces. Publishers request specific lengths for novels not because those are the best lengths to tell a story, but because they have carefully studied the finances behind producing books. A 500 page paperback costs very little more than a 200 page paperback, for example; the printing cost of either is minimal, so the cover price publishers would have to place on a 200 page paperback would be almost the same as the price they'd be able to put on a 500 page doorstopper. Readers get more feeling of value from buying a longer book - when you place the 500 page novel next to the 200 page novel on the shelf, both for $8, the 500 page one looks like the better deal. But bookstores can't place as many copies of longer books, which results in reduced orders for longer books.
So physical book length is determined by marketing forces: longer books having more apparent value for readers, but shorter books taking up less space on shelves and racks. The "happy medium" as it exists today is the result of significant (and ongoing) data analysis by major publishers.
In short: the medium, not the story, is determining the length of stories being told. Go back fifty or sixty years, and most books were half the length they are today, because the market pressures were different.
Ebooks free us from some of these expectations. As a result, short fiction is seeing huge renaissance right now; some writers are looking at perhaps being able to make a living from just short fiction for the first time in decades. Serialized stories are coming back in a BIG way; little 8-20k episodes released on a weekly, bi-weekly, or monthly basis. Reading material for an hour or two. Something like a TV episode you can read on the train headed into work and finish up over lunch or on the way home.
Short novels are back, hugely so. The old norm for a novel used to be 40-60k words or so, and writers are moving back in that direction. But there are longer works as well, because there is no *upper* limit on book size either. So writers can produce stories which are the length the story ought to be. However long that might be, for that story. And writers can now make sales of that work, be it 3k words, 10k words, 25k words, 60k words, or 500k words. (Although there is significant downward pressure now on story length - it is advantageous to release 4+ works per year, keeping something in the "new" category at all times, which is easier to accomplish if at least some of the works are shorter.)
What do you think? If you're a writer, is the digital medium impacting how you write? The length of your work, the pace of your work, what sort of stories you are willing to try? As readers, are you trying out new stories at different lengths via ebooks? Are you reading short stories? How do you feel about serials? If doorstopper-length novels start to go away, will you miss them? I'd love to hear your thoughts on the subject.