I don’t know where everyone gets the idea that the costs of publishing eBooks are zero. I see that comment all the time.
If you want someone to edit the book or prepare it for the zillion formats that eReaders use it comes with a price tag. That stuff doesn’t happen all by itself. If you expect the author to do that stuff you are going to see a lot fewer books coming out or coming out in limited formats. The biggest complaint most people have about eBooks is the poor quality of editing, so if you think you are going to selfpublish without the proper number of rounds of the editing process you are setting yourself up for failure.
Selfpublishing is growing dramatically and so is the eBook market. It's leading to a problem of differentiating the good from the bad. Some new filtering mechanism is likely going to develop to separate the chaff from the wheat, but I haven't seen it yet. There is still a lot of crap making it out into the online market. There are a lot of well-known authors making an effort to take control of their publishing but it does take a toll.
I think marketing is a huge issue, maybe the biggest. If you can't reach your audience you aren't going to sell any books. Having an actual marketing plan and following through on it will likely be the difference of success or failure of your book, and that holds true for eBooks and traditional paper books. Large publishing houses can market your book in a way that you will likely never be able to manage unless your name starts with J.K.
Publishers have their place in the scheme and I don’t think most authors want to do those things that publishers can bring to the table. It takes away time from writing and a lot of it requires a skill set they may not have. It's all a matter of finding the right pricing structure that will make the publisher a viable entity and still provide the author with the appropriate level of compensation for their efforts. Some of the publishers are figuring it out and moving in the right direction now. Time will tell if the big publishing houses will figure it out.