Accessibility is about making your applications usable by people with limitations that prevent or impede the use of conventional user interfaces (UIs). Karl Erickson just wrote a QuickStart with a bunch of tips to help you make your apps more accessible
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Read the complete post at http://blogs.msdn.com/b/silverlight_sdk/archive/2010/09/01/guidance-on-making-silverlight-apps-accessible.aspx