One of my favorite radio programs is the excellent Science Friday, which is broadcast on NPR (National Public Radio) in the USA. A recent edition talked about the plasticity of the human brain and included some interesting statements about research into treatments for dyslexia. I had not heard of the studies linking dyslexia to hearing difficulties - not vision processing difficulties as one might assume. You can find an MP3 download at the link above.
Clarification: I should have said "linking dyslexia to auditory processing difficulties" or something similar, not "hearing difficulties", which was too ambiguous. I should have also said "not only vision processing difficulties", as some theories of the roots of dyslexia still focus on vision processing (for instance, the so called magnocellular theory).
The root causes of dyslexia are not fully understood and serious research is ongoing. I suggest the following Nature article (Franck Ramus, 2001) for an overview.
I would also like to say "thank you" to the readers who pointed out my misstatement and the ensuing discussions in the comments. If you are just now coming across this post, I recommend reading the comments to catch up.