To be clear, there were two different electronic book things discussed at the seminar, and one more possibly hinted at.
First, they are releasing their back catalog of novels in e-book format (Kindle, Nook, etc.).
Second, James Wyatt started talking about an "electronic bookshelf" of all of their Eberron content in which customers would pay a subscription fee and get access to everything for a year. That's as far as he got before the other WotC people said, "Um, James, we haven't actually announced that yet." That's as much as we know right now. Is it e-book? PDF? Something else entirely? No clue.
Third, because they said that they're thinking about what they want to do with older edition content, there's speculation (including from me) that this Eberron thing might be a trial balloon for other older edition releases in some kind of subscription format. Details are obviously completely unknown, and I wouldn't expect timing to be any time in the next nine months, either. Just my own speculation based on sitting through the seminar.