Boxcar is our top rated, paid iPhone app that reads your Twitter stream (after authenticating with oAuth) looking for direct messages and @ replies.  It then pushes them to your device when found.  The downside of the service is that the application itself does not tweet, it launches whatever Twitter client you have installed also.  You get to choose between Tweetie, Twittelator, Twitterfon, Birdfeed, Twitterena, Twitterific and even Hahlo.

You get one service included in the purchase of the application, and adding on Facebook, Twitter searches and even email is $.99 per service.  So they definitely have a monetization plan involved.

Their stats on the page show they have pushed out almost ~13 million notifications, hopefully showing they are a stable and reliable service.