Twitter lead designer Doug Bowman, responding on Dribbble to a proposed more-Tweetie-like icon for the newly rebranded Twitter for iPhone:
The best version used more blue for the bird, but retained the exact same silver color of Tweetie’s icon. I thought it was the best choice of everything we had. For several reasons I won’t go into, we opted for the version the app uses now. via Dribbble
I didn’t upgrade Tweetie as long they keep this ugly blue icon.