The Streaming API has transitioned from Developer preview to Generally Available! What this means is that any org with API access also has Streaming Access enabled now. To get started, take a look at the following documentation and examples:
Lastly, if you’re local to Northern California or will just be in the neighborhood on March 15, don’t forget to sign up for Cloudstock 2012 at Moscone West! The Salesforce Platform API team will have an hour-long breakout session going over how to use the Streaming API to deliver updates in near real-time. The conference is FREE and is full of other great content. Sign up here!