If you have Scroll Lock on, pressing an arrow key won't move the cursor but will scroll the worksheet - but that doesn't seem to be your exact problem. I can't reproduce the issue so I don't think it's normal behaviour; I have 2010, so it's possible that it's a new 'feature' - but unlikely I think.
I know that doesn't help solve your problem (sorry ), but at least you know it's likely to be something specific which has changed.