We will test Streaming API Tweet Annotation as early as next week - June 7th through June 11th. All streaming clients, including user streams preview clients, should expect empty annotations and the rare populated annotation during arbitrary test periods. Assuming general stability, on our end and yours, we'll eventually leave annotation delivery turned permanently on. General purpose JSON and XML parsers should experience no problems with this change.
Expect a further announcement just before we flip the switch for the first time.
Infrastructure, Twitter Inc.