@grahamd: I occasionally have felt the pain of wrapping @classmethod (or @staticmethod). Never enough though to think of how to fix it. I really don't have the stomach to review your wrapt library, but your code looks okay except for style and missing tests. I'd also recommend adding a few words to the docs. (And yes, all of this is your responsibility -- nobody has time to do all that stuff for you.)

Style-wise:
- the continuation line in your patch is not properly formatted;
- either the else block should also use { } or the else clause should be omitted.