I'm using LoadOnDemand="xml" to load the rows as the user scrolls and handling AfterXmlHttpResponseProcessed on the client side so that I can show the user how many rows have been downloaded and how many available rows on the server.  The following is my event handler

 function AfterXML(gridName)
{
    var grid=igtbl_getGridById(gridName);
    document.getElementById('status').innerHTML='Loaded ' + grid.Rows.length + ' of ' + grid.RowsServerLength;
}

I've noticed with the RowsRange set to 200 and about 250 items on the server when I scroll down to the bottom it gets the remaining 50 but the Rows.length in the event is 400 not 250.  BTW I'm using 2007 v3.  For now I just check for this and use the RowsServerLength if Rows.Length is greater than RowsServerLength