We've released the first version of the Tracker API. It's a RESTful HTTP XML interface, uses a simple, token-based authentication mechanism, and allows you to query as well as update stories in your projects.
To get started with the API, read through the API Help page (it includes examples), and create a token on your Profile page.
We plan to enhance the API and add support for more operations and formats (including JSON) based on your feedback. Please let us know what you'd like to see added, either by email or via Satisfaction.
Two words - awe, some!