By way of a little history...
Back around version 1.0.9, we worked extensively with some good folks down at University of Texas at El Paso ( UTEP ). You may recognize the reference from our Guided Tour ( among other places ). We went through an extensive exercise at that time to get DNN to be Bobby508 compliant. That effort was led on our side by Bruce Hopkins.
There are certain characteristics of .Net that make it very challenging to reach any compliance ( whether that is 508, XHTML, etc ). Because certain controls, etc. actually generate HTML that cannot be modified ( without great difficulty ). So our implementation, even at that time, was not perfect.. but it was pretty darn good ( with the exception of the Solpart menu, in those days... which has greatly improved ).
I don't think that it would be fair to say this is not "important" to us. And the largest part of the burden has definitely shifted down to the skin designer. But there are probably a number of things we could be doing to help preserve the ABILITY to reach Bobby compliance in module developers, where the other large piece of the issue lies. I am imagining things like coding standards for modules that make it possible to stylize output HTML for compliance ( if so desired ), even if its not mandated out of the box.
If there were a champion to this cause... and mind you that championing requires significant personal time / effort... prioritization of this could change. But without that, its simply a question of resource availability for us. We definitely support the spirit of the ADA compliance guidelines.