When I bought my Nook, I discovered the world of publishers who don't produce physical books, but only e-books. I thought this was really interesting. Examples of these publishers are Carina Press (which is an imprint of Harlequin) and Omnific Publishing. The books typically sell for less than five dollars and the publishers put out several titles a month. I think these publishers are great because they accept unsolicited manuscripts, and they seem a bit more accessible than the 'big' publishers. There are also traditional publishers who accept unsolicited manuscripts, like many of Harlequin's imprints and Flux.
That said, I've been aware of the "big 6" publishers since I was a child, and I'd look at the spines of books to find out who published them, and I'd dream about getting published, it was on one of the "big 6" publishers, but now I've re-thought that goal. Don't get me wrong, I'd still love to be published by a large house, but I don't think it's the be all end all, especially for your first book. With the e-book revolution, you don't have to be published with a big house to be a best seller. I can think of a few authors who are self-published who are Kindle top sellers, and several more who are pubbed with small or e-book presses who sell well too.
I think the ways the publishing industry is splintering off is a good thing for the most part. Sure, there are small presses and self-publishers who rush product out before it's edited to the standards of the "big 6", but that's what user reviews and free samples are for. I also like the idea of publishing your first book with a smaller press, and then perhaps going the traditional route for your next books.
Have you considered being published/or are published by a smaller traditional press or an e-book only press? Why or why not?