Simple question, although the answer probably isn't...
Do Flash or Silverlight offer accessibility options?
Should we ever use these if we are trying to build a truly accessible site?
I don't know about silverlight but flash supports key board interactions and is meant to work with the latest versions of screen reader software. However older versions of screen reader software do not work well with flash.
The company I work for develop a lot of content which has to work with screen readers. Up until now we have never really trusted the accessible features of flash; we always provide a more accessible alternative.
However accessibility covers more than just screen readers and keyboard interactions it also covers what tech the user has to have to view the content. So if you don't have flash and silverlight the content would also be inaccessible.
My recommendation would be (if you have to use flash) to make sure your site degrades so that if your user is using a screen reader or they don't flash the content is available in another format.
So obviously there's lots of points of consideration in such a question - and could become a whole discussion.
But in the spirit of Q&A I say "No", we should not use these proprietary technologies and should begin to abandon them for anything that must take accessibility into account as a critical component to the goal of the project.
In short, I think the momentum of the web industry is so heavily focused on that standard right now (and for good reason) that going down the road it will be harder and harder to make a good case for use of technologies like Flash and Silverlight - at least in environments that must be truly accessible.
The answer is NO!!! You want the "Accessibility" and by this, you need to have your contents accessible for a wide range of users. Using a plugin like Flash or Silverlight, will reduce the accessibility because users who don't have these plugins, will see nothing!
For flash there is some attempt at accessibility. Here's a reference that is helpful. It says...
The Accessibility class manages communication with screen readers. Screen readers are a type of assistive technology for visually impaired users that provides an audio version of screen content. The methods of the Accessibility class are static—that is, you don't have to create an instance of the class to use its methods.
I haven't used it myself, but it came up during some research into Flash and accessibility. It looks like there's some hope. :)
Actually I just found this post on Stackoverflow to be enlightening. The person who posted this is visually impaired and gives good info about Flash and accessibility.