Split cell keyboard shortcut wired up. Merge markdown adds 2nd \n.

IPython/html/static/notebook/js/keyboardmanager.js

@@ -241,6 +241,11 b' var IPython = (function (IPython) {'
             // Undo last cell delete = z
             notebook.undelete();
             return false;
+        } else if (event.which === 189 || event.which === 173) {
+            // how fun! '-' is 189 in Chrome, but 173 in FF and Opera
+            // Split cell = -
+            notebook.split_cell();
+            return false;     
         };
         // If we havn't handled it, let someone else.
         return true;

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

@@ -1079,7 +1079,7 b' var IPython = (function (IPython) {'
                 cell.set_text(upper_text+'\n'+text);
             } else if (cell instanceof IPython.MarkdownCell) {
                 cell.unrender();
-                cell.set_text(upper_text+'\n'+text);
+                cell.set_text(upper_text+'\n\n'+text);
                 cell.render();
             };
             this.delete_cell(index-1);
@@ -1109,7 +1109,7 b' var IPython = (function (IPython) {'
                 cell.set_text(text+'\n'+lower_text);
             } else if (cell instanceof IPython.MarkdownCell) {
                 cell.unrender();
-                cell.set_text(text+'\n'+lower_text);
+                cell.set_text(text+'\n\n'+lower_text);
                 cell.render();
             };
             this.delete_cell(index+1);