In the beta welcome email, it says this:
Quick Start
To use Let's Encrypt's official client to obtain your real certificates, you will need to provide the production API URL on the command line:
https://acme-v01.api.letsencrypt.org/directory
When running the Python client (installation directions [1]), be sure to specify the --server argument with the production URL:
Upon initial read, it sounded like there was both a "Python client" and an "official client." It's not clear that's there's only one client / these are the same thing.
Paul