I think to keep many contributions it's important to keep the code as free as possible. GPL seems a bit restrictive causing two issues:

• released apps on the app store must give their source code. If I understood GPL: an app (software) != web site (service). Some university/company won't reveal their additional source code.
• GPL seems to clash with the app store: http://www.zdnet.com/blog/open-source/no-gpl-apps-for-apples-app-store/8046

I think something like MIT license seems good to me (http://jordanopensource.org/freeplanet/article/open-source-license-used-widely-app-store-apps)