To create a basic Silverlight control that integrates with SharePoint Online via Web Services that can then be deployed to SharePoint Online; you need a SharePoint Online account with at least one site collection. You must have “full control” or higher permissions on the site collection used for this example. You also need Microsoft Office SharePoint Designer 2007 and Microsoft Visual Studio 2008. Lastly, these instructions assume the Silverlight SDK and the Microsoft Silverlight Tools for Visual Studio 2008 are both installed on your machine.

Full Article: MOS Blog