Speaking to E-Access Bulletin after the event Mark Bide, Executive Director of EDItEUR, said accessibility is “rising up the agenda” in the publishing industry. He said there are now good levels of compliance with a 2010 recommendation by the Publishers Association that text-to-speech be routinely enabled on all e-books across all platforms, except where there is an audiobook edition commercially available.
Important stuff, but not all e-book platforms are created equal. Though I've been unable to easily locate a completely up-to-date comparison, this from 2012 gives a pretty good overview of which platforms (hardware and software) are doing well for accessibility issues, and which need to try harder. This from 2011 is less recent but more detailed.
I've not had enough experience building completely accessible e-books yet, but my initial tests with iBooks Author for iPad indicate that it's pretty straightforward to add assistive features to iBooks as you build them, and the iPad's built-in assistive features are outstanding in mobile technology.
If anyone has more up-to-date information or experience I'd love to hear, and share, it.
Amazon announced some improvements to the accessibility of their Kindle Fire products in December, no doubt they "heard from thousands of customers who are vision-impaired": Those previous studies had nothing good to say about assistive features on modern Kindle devices.