A year ago I took a seminar in Milan where I spoke on the future possibilities of XML when applied to blogs (you can see the slides here). Many people got impressed when I showed them how to create a blog entirely written in XML, transformed with XSLT and displayed with CSS.
In the meantime, I had a nice conversation with Livio Mondini, who pointed out some interesting remarks on my conference. First of all, XML poses some accessibility problems when it comes to screen readers. In fact, since XML has no predefined DTD, screen readers don't know in advance how to handle XML elements. Simply put, a screen reader will ignore the element together with its content.
I didn't test extensively what happens when you transform an XML document with XSLT. This would be a useful test to do. Theoretically speaking, a screen reader should be able to recognize the transformed XML element but only when the target format is HTML (which, of course, has a predefined DTD). However, since screen readers read only what a browser reads (that is, the output provided by a web browser), what happens when a browser doesn't support XML and XSLT properly? That's another interesting question.
In other words, if the current browser is Internet Explorer 6, which by default doesn't read XML unless you activate the correct linking to the MSXML library, what a screen reader will read? I can't test it myself because I do not have a screen reader installed on my computer. I leave this test to you. If you find out something interesting, please let me know.