I used following sample using Q4 2006 version of Telerik controls. It used to work fine until we upgraded to Q2 2008. Debugging through FireBug in FireFox, I realized that CallBack function is no longer supported. I searched for replacement function and came across _invokeDialogCallBackFunction and it works fine, but wanted to make sure. Now I changed my code according to your suggestion and it is working fine. Thanks.
var oWindow = GetRadWindow();
//Variant1: Invoke the calback function, pass it the result as an argument
oWindow.CallBack(updateValue);
oWindow.Close();
//Variant2: Passing the argument to the Close method will result in the same behavior
//oWindow.Close(oNewText);
//Variant3: Possible to set the Argument property of RadWindow here, and read it in the OnClientClose event handler!
//oWindow.Argument = oNewText;