Hi,

Based on our understanding, the IsInDesignMode method checks for the current application due to a known issue in Cider with Silverlight 3, where the DesignerProperties.GetIsInDesignMode method doesn’t work correctly. You can find more information and possible workarounds about this in the following work item:

• Region manager in V2 fails to recognize non-WPF applications
  

Regarding if this is a show stopper for using Prism, no it isn't.

Regards,

Federico Martinez
http://blogs.southworks.net/fmartinez