Everything lives under /v1. Inside /v1, changes only add things: a new call, a new optional field, a new field in a response, a new event type. So ignore fields and event types you don't recognize, and your code keeps working.
Anything that would break working code goes in /v2. When that happens, we try to keep /v1 working for at least 6 months after the notice here. We may move faster for security, for legal reasons, or if we shut BuddyPad down.
The API is free for now.
First public release: tokens, goals, buddies, comments, nudges, check-ins, suggestions, the events feed and webhooks.