In the case of NSDictionary, Underscore.m's map iterates over objects and keys.
Fast Enumeration only iterates over the dictionary's keys, when you probably want the value or both.
But I agree, supporting any class that implements NSFastEnumeration would be nice to have.
You may already know this, but you can also enumerate over NSDictionary entries using enumerateKeysAndObjectsUsingBlock: [1] and enumerateKeysAndObjectsWithOptions:usingBlock: [2].
The latter giving you the ability to enumerate in reverse order or spread the enumeration across multiple cores using Grand Central Dispatch.
But I agree, supporting any class that implements NSFastEnumeration would be nice to have.