My father-in-law is slowly losing his eyesight to glaucoma.
If Apple sort out iTunes handling of mp3 talking books, and if they make buttons/wheel a little more tactile, I could see it being a good bet for him.
I nipped over to the RNIB website and the cost of some of the products is scary. A mass market device could easily do the job... And it removes some of the stigma associated with buying "special" products.
And done right, it would enable navigation of menus without taking the iPod out of your pocket. There are places and times when I don't want to show off my kit. Also, if I'm exercising, I don't neccesarily want to stop and look at the iPod to adjust it. I can generally slow down and spare a hand. Jogging, I need to keep my eyes open for dogshit, uneven pavements and inconsiderate cyclists on the pavement.
Similarly, if you could get the Touch to show big text instead of album flow, that might be good too.
This sort of stuff costs manufacters nothing. But do they do it? No.
Jobs hate because could do better.