Nice article. It's a very nice use of dynamic proxies to separate concerns. One thing you may wish to consider as part of your "fully featured version of Memoizer" is a WeakHashMap for your top-level map that maps Method instances to value caches. Using a "standard" map implementation will prevent the JVM from unloading any class that has a method as a key in your top-level map.
Alternatively, if you want to allow the JVM to unload classes but keep cached values for a method, you could use the fully-qualified "mangled" method names as the keys.
Of course, as long as you keep a reference to the dynamic proxy, you keep a reference to an instance of a class, which means the class wouldn't be a candidate for garbage collection anyway.
Just something to think about.