Added some comments to the Alt-Enter code part, and also added checking whether a new code cell is really needed.

IPython/frontend/html/notebook/static/js/notebook.js

@@ -115,8 +115,12 b' var IPython = (function (IPython) {'
                 that.execute_selected_cell();
                 return false;
             } else if (event.which === key.ENTER && event.altKey) {
+                // Execute code cell, and insert new in place
                 that.execute_selected_cell();
-                that.insert_cell_above('code');
+                 // Only insert a new cell, if we ended up in an already populated cell
+                if (that.get_selected_cell().toJSON().input !== "") {
+                    that.insert_cell_above('code');
+                }
                 return false;
             } else if (event.which === key.ENTER && event.ctrlKey) {
                 that.execute_selected_cell({terminal:true});