noblocked is an iPhone 3.1.3 app which automatically rejects incoming calls from callers that are hidden (or shown as blocked by the iPhone).
All the other calls aren't affected.
This app is not a generic whitelist / blacklist caller app, although it could be easily extended to do that too.
I wrote the app for personal use but I'm releasing its source code so that other people may learn from it -- I know it took me a while to get used to notions like private frameworks, etc. since I've always used only the official, documented, APIs.
Obviously, I'm not going to submit this app to the AppStore.
A precompiled binary is available in the downloads section but I haven't actually tested it, it's just an unsigned build I've uploaded. Clarifications or some testing would be nice.