In thinking about how and why libraries would let patrons choose their content output it becomes apparent that the broader web trend of increased personalisation could allow libraries to offer much more for people with vision and or hearing impairments.
At the moment libraries have to buy pre-formatted content from publishers. For example, you buy the print version of the book, and then you have to separately buy the audio book, large print version, and the ebook. If you are dealing with internal or locally produced content (e.g. oral or written memoirs in local history collection) and you want to make it available in different formats, you need to reformat in house. Not surprisingly, for most libraries, content suitable for people with vision or hearing impairments represents only a small part of their overall collection.
BUT,... if the ReadWrite Top 5 Web Trends for 2009 becomes mainstream, there may be scope for library clients to pick the output of their choice. If data is more structured and more and more things are tagged, content could be reformatted on the fly. This could open up much more of the collection and give patrons real choice. For example, patrons could choose if they want to read an book on a screen, printed it out, or a computer read it to them.
The does mean that the content has to be formatted to begin with, but as with XBRL for business reporting, the principle is that you format it once and then open it up to be used and reused in many different ways. Of course there would be copyright and content licensing issues that need to be addressed.
There is certainly a lot more software out there that can read xml, or listen to audio and render it as text, See following for a few examples.