diff --git a/IPython/html/static/services/config.js b/IPython/html/static/services/config.js
index 0d9d559..0d2a9f9 100644
--- a/IPython/html/static/services/config.js
+++ b/IPython/html/static/services/config.js
@@ -48,7 +48,14 @@ function($, utils) {
         });
     };
     
+    /**
+     * Modify the config values stored. Update the local data immediately,
+     * send the change to the server, and use the updated data from the server
+     * when the reply comes.
+     */
     ConfigSection.prototype.update = function(newdata) {
+        $.extend(true, this.data, newdata);  // true -> recursive update
+        
         var that = this;
         return utils.promising_ajax(this.api_url(), {
             processData: false,
@@ -97,6 +104,24 @@ function($, utils) {
         return this._class_data()[key] || this.defaults[key];
     };
     
+    /**
+     * Set a config value. Send the update to the server, and change our
+     * local copy of the data immediately.
+     * Returns a promise which is fulfilled when the server replies to the
+     * change.
+     */
+     ConfigWithDefaults.prototype.set = function(key, value) {
+         var d = {};
+         d[key] = value;
+         if (this.classname) {
+            var d2 = {};
+            d2[this.classname] = d;
+            return this.section.update(d2);
+        } else {
+            return this.section.update(d);
+        }
+    };
+    
     return {ConfigSection: ConfigSection,
             ConfigWithDefaults: ConfigWithDefaults,
            };
diff --git a/IPython/html/tests/notebook/dualmode_cellinsert.js b/IPython/html/tests/notebook/dualmode_cellinsert.js
index 1764042..093f5b6 100644
--- a/IPython/html/tests/notebook/dualmode_cellinsert.js
+++ b/IPython/html/tests/notebook/dualmode_cellinsert.js
@@ -33,7 +33,7 @@ casper.notebook_test(function () {
     });
     
     this.thenEvaluate(function() {
-        IPython.notebook.default_cell_type = 'selected';
+        IPython.notebook.class_config.set('default_cell_type', 'selected');
     });
     
     this.then(function () {
@@ -47,7 +47,7 @@ casper.notebook_test(function () {
     });
     
     this.thenEvaluate(function() {
-        IPython.notebook.default_cell_type = 'above';
+        IPython.notebook.class_config.set('default_cell_type', 'above');
     });
     
     this.then(function () {
@@ -61,7 +61,7 @@ casper.notebook_test(function () {
     });
     
     this.thenEvaluate(function() {
-        IPython.notebook.default_cell_type = 'below';
+        IPython.notebook.class_config.set('default_cell_type', 'below');
     });
     
     this.then(function () {