3 points:
1. The Appstore is not a public forum, and Apple can sell whatever the hell it wants on it. This is not an issue of freedom or of speech. It's an issue of the market. If you're pissed, go buy a Droid.
2. This action by Apple (if it IS an action, and not just some glitch or hack; there hasn't been an announcement) is not discriminatory against Christians. If an atheist organization put out a "Christian Cure" app, you better believe that Apple would block or pull that one, too.
3. We need to stop treating the idea that "homosexuality is a disease that is curable" as just another opinion and call it what it is: hate speech. It belongs in the same category of garbage as arguments about the genetic inferiority of certain races or genders. You have a constitutional right to say it, I suppose, but nobody, including Apple, has to take you seriously.