Leonid Mamchenkov has a quick post to his blog showing how to enable and disable (well, restrict from enabling) CakePHP plugins on the fly - no need to modify config files if you only need it for a certain request.
I am currently working on a rather large project which is based on CakePHP framework. In order to simplify the task, I've broken down the whole application into a number of CakePHP plugins. Now, however, I want to enable/disable plugins on the fly. After a brief search around I couldn't find how to do that. Asking a question at #cakephp IRC channel did it. RabidFire instantly replied with the link that gave me an idea. 30 seconds later I had a working solution.
Since the plugins extend the main AppController, all it took was a beforeFilter() method call that looks at a request parameter for the plugin name, checks against an "allowed" array and throws an exception of it's not found or can't load it.