- If you’re developing a managed .NET client application—for example, a WPF application—you need to add references to Microsoft.SharePoint.Client.dll and Microsoft.SharePoint.Client.Runtime.dll. You can find them in the 14\ISAPI folder on your SharePoint server.
- If you’re developing a Silverlight application, you need to add references to Microsoft.SharePoint.Client.Silverlight.dll and Microsoft.SharePoint.Client.Silverlight.Runtime.dll. You can find them in the 14\TEMPLATE\LAYOUTS\ClientBin folder on your SharePoint server.
I hope this saves someone the trouble of trawling the SharePoint root looking for assemblies!
Update 10th March 2011: I keep reading that the Silverlight client assemblies are in the 14\ISAPI\ClientBin folder. This folder doesn't exist in any of my installations.