Boxcar [$1.99 - iTunes link] is almost more enabler than app. It only does one thing, but thus far it does it very, very well -- it sends Push Notifications for Twitter @mentions and Direct Messages (DMs) that then open in either Tweetie or Twitterrific.
Setup is straight forward; you log into your account through an in-app view of Twitter's website, and authorize Boxcar. You can select whether you want Push Notification for either @mentions, DMs, or both, and which of the aforementioned apps you want to open them in (hopefully support for more options will be forthcoming, such as Tweetdeck, Birdfeed, Twittelator Pro, etc.). You can also choose to automatically tweet them a shout out.
Like all Push-enabled apps, you can exit to the iPhone Settings to turn on or off Sounds, Alert (text boxes), and/or Badges.
That's it. Sit back and enjoy Push Notifications so you can immediately know when anyone talks to or about you on Twitter. Even when you're watching a movie on your iPhone. Or playing a game. Or writing an SMS. Or even taking a phone call -- if anyone still does that...
In other words, it works so well, you may have to learn to better manage your Twitter interruptions.
Now the crux: is a couple of bucks a lot to pay for an app that arguably only extends the usefulness of other apps that you already paid a few bucks (or more) for? Depends on how badly you want Push Notification for Twitter, whether you're willing to wait for full-on Twitter clients to add it themselves (however long that takes), and if you realize a couple of bucks is very little, and partly going to support the developers servers that are handling the Twitter calls and passing them on to Apple's notification servers.
I bought it without a second thought. You'll have to weigh the value (which is more important than cost) yourselves, and let me know what you think in the comments -- or @reneritchie and I'll get it right away via push ;)
More pics after the break!