In design time, binding to and object datasource returns a "ID could not be set to property DataFieldID". However, if I run the application it works fine.
It was related to assembly cache. I deleted Telerik.Web.UI.dll from applications/bin directory and added C:\Program Files\Telerik\RadControls for ASP.NET AJAX Q2 2009\Bin35\Telerik.Web.UI.dll to GAC, and this solved the problem.