I've just come across an ebookstore today that specializes in digital comic. Izneo were formed as a partnership of 12 comics publishers, and they have over 1000 titles on hand and they're adding a 100 new titles each month. The site is based in France, and they sell ebooks in English and French from quite a few European publshers including: Bamboo, Casterman, Dargaud, Delcourt, Dupuis, Glénat, Le Lombard et Soleil, and others.
Right now you can only rad online in a Flash based web app, but they plan to add support for ereaders. Prices start at 1.99 € for a rental, and most comics are selling at 4.99 €.