Does your favorite Twitter application not support Push notifications? Probably not. This is where a third party can step in and provide a specific service. This is what Boxcar does.
Essentially Boxcar is an intermediate app that polls your twitter account every couple of minutes and then sends a push notification to your iPhone or iPod Touch that you have a new Direct Message or Mention (@reply).
When a new message is detected, you will see the message and get an audible notification that you have received one. Don't worry, you can easily disable this to keep it from annoying you if you are popular twitter user.
Once you click on the notification, it will launch your twitter client of choice. Currently Boxcar supports Tweetie, Twitterrific, Twittelator, Twitterfon Lite, Twitterfon Pro, and Twinkle. Boxcar is even granular enough to use combinations of apps with @messages opening one app and DMs opening another. It's your choice.
If you open up the Boxcar app, you can even type a DM to someone and if they are a Boxcar user it is pushed to them instantly, negating the whole polling every couple of minutes. While this is an added feature, I double it will be heavily used by most, as they will just DM from their regular twitter client.
Boxcar works as advertised, right now. My biggest concern is that the developer has to keep his server running and be able to grow with it as the Boxcar user base increases. If it gets extremely popular, growing is no small feat. I love that Boxcar is also one of the only services that doesn't charge a small subscription fee.
Conclusion: I think Boxcar is a cool little app and brings push notifications to my iPod/iPhone when other twitter clients can't.While this is great on the initial purchase for $2.99 and that's all you pay... if he ever decides to let the server lapse, it goes down, etc... so do your twitter Push notifications. On the flip side, for $2.99 who cares