Mac OS X provides a pretty nice data-binding technology for developers, called Cocoa bindings. Unfortunately, the Cocoa bindings mechanism is not available to iOS developers, so iOS developers have to spend a lot of time writing code to keep user-interface elements and data in sync.
However, while Cocoa Bindings is not available on iOS, the underlying key-value coding (KVC) and key-value observing (KVO) mechanisms are, so it is straightforward to implement your own poor man's data-binding mechanism and eliminate some of the drudgery.
I'm not the only one to try this. Here are a few similar projects I found on GitHub: