In this article you will find how to bind an Excel spreadsheet (.xlsx) to datagrid using Silverlight 3.
I use a class to transform each dictionary key into a property of anonymous typed object this came from and is downloadable (http://blog.bodurov.com/How-to-Bind-Silverlight-DataGrid-From-IEnumerable-of-IDictionary). Also use an unzipped which is very handy from (http://www.sharpgis.net/post/2009/04/21/REALLY-small-unzip-utility-for-Silverlight.aspx). This blog also shows how to get around the header issues which are are an issuse in the first blog (first URL) by binding different data to the key of the dictionary and that is used in the header itself.