It looks as they welcome serious development contributions, here's the github of TextSecure-iOS:
https://github.com/WhisperSystems/TextSecure-iOS
They also experiment with alternative currency contributions:
https://whispersystems.org/blog/bithub/
I think what I've never found is a honest description how much exactly we have to trust their server (on various levels, from the security aspects to the uptime aspects), and what would be the possibilities for involving other servers.
Even if you don't feel iOS devices are trustworthy, as long as it's limited to the Android-only world, it's not going to gain a wide adoption.