No need to run trim on every class iteration in addClass, thanks for the heads-up Leeoniya.

src/attributes.js

@@ -40,12 +40,13 @@ jQuery.fn.extend({
 						elem.className = value;
 
 					} else {
-						var className = " " + elem.className + " ";
+						var className = " " + elem.className + " ", setClass = elem.className;
 						for ( var c = 0, cl = classNames.length; c < cl; c++ ) {
 							if ( className.indexOf( " " + classNames[c] + " " ) < 0 ) {
-								elem.className = jQuery.trim( elem.className + " " + classNames[c] );
+								setClass += " " + classNames[c];
 							}
 						}
+						elem.className = jQuery.trim( setClass );
 					}
 				}
 			}

test/unit/attributes.js

@@ -438,7 +438,7 @@ test("val(Function) with incoming value", function() {
 });
 
 var testAddClass = function(valueObj) {
-	expect(4);
+	expect(5);
 	var div = jQuery("div");
 	div.addClass( valueObj("test") );
 	var pass = true;
@@ -460,6 +460,10 @@ var testAddClass = function(valueObj) {
 	div.attr("class", " foo");
 	div.addClass( valueObj("test") );
 	equals( div.attr("class"), "foo test", "Make sure there's no extra whitespace." );
+
+	div.attr("class", "foo");
+	div.addClass( valueObj("bar baz") );
+	equals( div.attr("class"), "foo bar baz", "Make sure there isn't too much trimming." );
 };
 
 test("addClass(String)", function() {