Colloquy for iOS does exactly this. Then again, you can't install a binary on your iOS device without a developer license, so I guess this model can't be replicated outside of iOS.
I do this too with some of my iOS apps. I put them GPL licensed onto github and anyone with the knowledge (and dev license from Apple) can get the apps practically for free.
But I'm not really vocal about that. If someone finds the source: great for them. If not great for my wallet ;)
