I can add online maps/layers to Desktop 10 if I add them from ArcMap using Add Data, but if I'm looking at a map in ArcGIS Online and I click the link "Open this map in ArcGIS Desktop 10" it doesn't work. I get the "Opening item.pkinfo" dialog and it defaults to ArcGIS File Handler and I click OK. Then I get a "could not open the specified file error" every time. If ArcMap is closed, it does start the application, but then the error appears. If ArcMap is open, I just get the error.