I used to cook beans without salt, too. I saw an episode of Good Eats years ago that talked about it, he tested it by cooking identical batches of beans with and without salt, with no difference in cooking times. (At least, I think it was Good Eats, it was at least 5 years ago so I don't remember for sure.) After that, I stopped worrying about adding salt to beans, and notice no difference in cooking times when I do add salt. I haven't done the experiment with two batches with and without salt. On the occasions when I don't specifically add salt with spices at the beginning, I almost always at least throw in a ham hock or ham or bacon skin, all of which contain a significant amount of salt, and in all scenarios my beans cook within 2 or 3 hours on low heat.
I also just read a little blurb in Saveur magazine about this very topic, the writer was surprised at one of the featured recipes calling for adding salt to the beans right at the beginning, but tried it anyway and they came out perfect. It says, in part, "And the salt? Adding it didn't change the texture of the beans or alter the cooking time, so salt as freely as your taste dictates. A final revelation: for one version we tried, we removed the tomatoes and noticed that the beans cooked a lot faster. Acidic ingredients, it turns out, slow down the cooking process dramatically. Who knew?" from the June/July 20009 issue,
http://www.saveur.com/article/Mise-en-Place/Good-Beans (it also talks about their experiment involving cooking times with soaked vs. unsoaked beans).