Found the solution.
EndInvoke would always throw "Callback called twice:" exception
If interested, below is the solution:

AsyncResult ar=(AsyncResult)iar;
Download del=(Download)ar.AsyncDelegate;
_censusDataStream=(CensusDataStream)((IMethodReturnMessage) ((AsyncResult)iar).GetReplyMessage()).ReturnValue; //DOES THE  TRICK

//_censusDataStream=del.EndInvoke(iar); DOES NOT WORK- bug?
				
_ds=this.processStream(_censusDataStream.oStream);
FireStatusChangedEvent(DownloadStatus.NotDownloading,_ds); 

thanks,

Vlad