This should absolutely be implemented as **:

- It is pretty much standard. Recursive globbing is not supported everywhere, but when it is, ** is used.

- A separate function will not allow me to let the *user* to decide when recursion should be used. I find this most important. When I need to find files internally, I always do so using os.walk etc. When I use glob, it is because I want to provide an interface to my users. 

- The change to support ** is actually pretty trivial. I have implemented this as a module here: https://github.com/miracle2k/python-glob2/

- It's backwards-compatible - or close enough anyway. ** is currently perfectly nonsensical, making it a meaningful syntax element is acceptable, I think.