I have been updating soem Silverlight 1.0 demo's to Silverlight 2 and came across this regular error. Its caused by a duplication of namespaces (e.g. UserControl redefines the namespace for its Canvas)

Fix is therefore pretty easy:

1) You need to search for any xmlns attributes in your XAML and delete them

http://www.andybeaulieu.com/Default.aspx?tabid=67&EntryID=94

2) Clean or Rebuild in Studio

http://learnwpf.com/Posts/Post.aspx?postId=86eeea5c-b174-45aa-9693-dbf55213fafa