From the announcement on the Sparrow website: (edit: now gone)
… we’ll be busy with new projects at Google, we do not plan to release new features for the Sparrow apps.
Best case scenario: We’re about to get an awesome native Gmail client for OS X and iOS.
More likely – We’ll get a decent iOS app, and the desktop app will never be heard from again.
The sad fate of Tweetie may be coloring my outlook, but I think this has zero percent chance of being a good thing for the users of Sparrow.