This is more a question for Microsoft than Esri. As long as Microsoft supports the tools that are used to develop and use Silverlight applications on Windows XP, Esri will continue to support Silverlight in the same capacity. You can view the latest system requirements for ArcGIS Silverlight developers and clients here: http://help.arcgis.com/en/webapi/silverlight/help/index.html#/System_requirements/01660000000t000000...
That said, if you have the opportunity to move to Windows 7 now, I recommend making the move.
