Amazon has announced a new category in its Kindle book store called "Singles," for works longer than a magazine feature but not quite as long as a full book. The submission process and pricing haven't been announced, but I like the idea. It fits a category of writing that's hard to place, say, like a certain blogger's 85-page graduate thesis that doesn't condense well and doesn't have the legs to stretch into a book.
But seriously, it's the first time I've seen any publisher take advantage of the digital format besides just tacking on multimedia extras. The length of what can be considered publishable has always been determined by the economics of print. A magazine piece could only take up so much space, and it's not worth printing a book unless it has enough pages. With digital content length doesn't matter, and this could be a real sweet spot for scholarly work, narrative nonfiction, and what we used to call novellas.