I am already quite fed up with my Birdsong Twitter App already. Hopefully, CarbonWP can reach the Windows Marketplace. From my understanding, it was develop for WebOS and now the developer has made one for Windows Phone that make full use of Mango features.
Here is a video that shows a few features of the app. I like the fact that it can pin Twitter list or new message on to the main menu. Hopefully, it can have instantaneous update when someone mentions you or send you a message. Currently, I only receive the update 1 or 2 days after someone mentioned me. I have to rely on Windows Phone notification to see that (but not the direct message).