ConcurrentDictionary<WeakReference, CachedData> is incorrect in this case. Suppose we are trying to cache info for type T, so WeakReference.Target==typeof(T). CachedData most likely will contain reference for typeof(T) also. As ConcurrentDictionary<TKey, TValue> stores items in the internal collection of Node<TKey, TValue> you will have chain of strong references: ConcurrentDictionary instance -> Node instance -> Value property (CachedData instance) -> typeof(T). In general it is impossible to avoid memory leak with WeakReference in the case when Values could have references to their Keys.
It was necessary to add support for ephemerons to make such scenario possible without memory leaks. Fortunately .NET 4.0 supports them and we have ConditionalWeakTable<TKey, TValue> class. It seems the reasons to introduce it are close to your task.
This approach also solves problem mentioned in your update as reference to Type will live exactly as long as Assembly is loaded.