You should cast rowKey to Number and use appropriate methods in case you use your own model which extend ScrollableTableDataModel.
pos = ((Number)argSelection.getKeys().next()).intValue;
In other cases post your page sources at first. And we try to help you.
Wow, that did the trick. Now the code works with RichFaces 3.3.0, too. I'm not quite sure what happened there, but I'm running out of time on the project, so no time to debug this further. Thanks a lot for the help! :-)