While the online demos of the RibbonBar control may look like MS Word and Paint, they are actually just mock-ups of those applications and only illustrate the RibbonBar control - the rest is not functional. As for how to develop a Word-like component - this is far from being a trivial task and would require a lot of work.
We are currently working on a native Silverlight rich text editor which will be available later this year, but of course won't have all of Word's features. As far as I know, there is no fully-featured Word-like component for Silverlight on the market today.
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