DC Comics has been publishing titles on the iPad for a while now, both in the official DC Comics app and in the Comixology app (which, as you may know, actually produces the official app anyway). But DC is now finally turning to iBooks to sell comics as well, with an iBooks version of the Batman: Earth One story that passed through the official Batman lines recently. This is DC's first title on the iBookstore, and it joins a slew of Marvel titles, as well as quite a few independent titles already selling comics in this way.
Most of the "big" line titles in the iBookstore are trade paperbacks -- bigger collections of Marvel and other titles that combine more than one monthly comic together. But that's not necessarily the only option out there: Some companies have released single issues on the iBookstore, available for just a buck or two per issue. Because the market is so open, these companies can do whatever they want.
Obviously, releasing titles in this way helps comic companies dodge the extra fees behind using Comixology's software to sell their books, but then again they miss out on Comixology's significant audience and discovery features that bring in new readers to old and obscure books. These days, comics companies will likely try whatever they can to sell these digital copies, and we'll see which method works best in the future.