There is some evidence. If you look at Apple, iBooks is totally accessible using the iPod touch, iPads, and these sorts of things. But then if you also look at Amazon, the Kindle is completely inaccessible. They've taken steps backwards. The Kindle Fire, which recently came out, did not include any accessibility, so I'm not really that optimistic.
We're going to need either publishing incentives—subsidies to publishers who produce accessible formats—or some other kind of legislation to require it to happen.