From their site...
The Fitbit API allows developers to interact with Fitbit data in their
own applications, products and services.
While currently in BETA (as in... we're still working on it), the API
allows for most of the read and write methods that you will need to
support your application. If you have uses or needs that are not
currently supported by the API, drop by the Dev Forum and let us know!
We look forward to working closely with the development community to
make the Fitbit API a system that enables you to do awesome, mind
blowing stuff