I note that Harlequin Enterprises, known for romance novels, will launch a new ebook line in April, 2010. This "imprint" will of course publish romance novels, and also science fiction, mystery, mainstream, fantasy, horror, thriller/suspense, and every other genre. I think this well-established publisher has the potential to create an effective presence for ebooks.
The publishing operation is called Carina Press. To submit manuscripts and for other info see http://www.carinapress.com/