Like all apps for the Zune HD thus far (including the recent batch of 3D games), we expect that the Twitter app will be free to download and include a momentary ad banner each time the app is launched. Currently, Zune HD users are able to use the device's integrated Web browser to access and update Twitter but have no way to cache the communications offline.
Unfortunately, Facebook updates will still need to be done the old fashioned way (through the browser) until an app materializes. With 15 days left in 2009, there's still a chance Microsoft will pull this one off in the nick of time. Granted, Apple will have reached its trillionth app by that point, but the effort will not be lost on the Zune faithful.