>Try

SET @sql_string = REPLACE(@sql_string, '@@idoc', CAST(@idoc AS varchar(10)))
>EXEC (@sql_string)

BTW, why it's @@idoc in the @sql_string?
>

That won't really help either... it works like this;

CREATE procedure dbo.getdata_wayne
@doc as ntext

AS

declare @idoc int
	-- Parse XML String	
	EXEC sp_xml_preparedocument @idoc OUTPUT, @doc
	-- Get Data
            SELECT * 
              FROM OPENXML (@idoc, '/ROOT/ID_S',1) WITH 
               (KEYVALUE varchar(16)) AS JT, PREMIUMHEADER
                WHERE PREMIUMHEADER.CPOLICYNUM = JT.KEYVALUE
	-- Remove XML document		
	EXEC sp_xml_removedocument @idoc
GO

Calling it with

exec getdata_wayne '<ROOT><ID_S KEYVALUE="99WCAL138462"/></ROOT>'

But because I'm creating a dynamic where, select list, etc, I don't have the same select.