but it doesn't work with me (ArcGis Server 10 + SL 4) - when i add Silverligth ESRI.ArcGIS.Client.dll i got error - System.IO.FileNotFoundException: Could not load file or assembly 'System.Runtime.Serialization, Version=2.0.5.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e' or one of its dependencies.
Try by adding a reference to System.Runtime.Serialization.
it has already been added but version is v4.0.30319 ( i use SL 4 + Net Framework 4)
Your project can not target both SL4 and .Net Famework4.
If you target .Net Framework 4, you can't use the Silverligth ESRI.ArcGIS.Client.dll, you have to use the ArcGIS API for WPF dlls.
What libraries i must add to the MyProject.Web?
This project targets .Net Framework 4 (I guess), so you have to reference the WPF version of ESRI.ArcGIS.Client.dll