I had to do this to pick from an array of classes exported from the Flash IDE Library. Unfortunately getting the class names from the library is not dynamic-- gotta write them out ..doing it this way at least.
_ http://www.kirupa.com/forum/showthread.php?t=292617
_ http://thillerson.wordpress.com/2007/03/01/runtime-class-instantiation-in-actionscript-30/

var i:int = 0;
var class_arr:Array = ['ClassA', 'ClassB', 'ClassC'];
var class_string:String = class_arr[i];
var class_type:Class = getDefinitionByName(class_string) as Class;
var class_instance:MovieClip = new class_type();

More on Class referencing
_ http://www.mikechambers.com/blog/2006/06/22/actionscript-3-get-a-class-reference-by-class-name/

SWF Class Explorer script
_ http://flassari.is/2008/07/swf-class-explorer-for-as3/

More on Embedded Assets
_ http://yourpalmark.com/2007/03/31/associating-custom-as3-classes-with-embedded-assets/

Unfortunately there's an issue with dynamically accessing symbols within embedded swfs
_ http://www.bit-101.com/blog/?p=864
_ http://www.bit-101.com/blog/?p=864#comment-28081
_ http://www.bit-101.com/blog/?p=864#comment-299425