If you attended the Spring '11 webinar, you already know that the REST API was to be included as part of the release. With the first round of production nodes upgraded this weekend, we have started transitioning from developer preview only to general availability in production. Woot! This means if your production org wasn't one of the lucky ones enabled this past weekend, then it will be this coming weekend.
You can always refer to trust.salesforce.com for release schedules.
To find out more about the REST API and how to use it with your organization, go here.