I’m finally exploring ARC (Automatic Reference Counting) for development on iOS 4 and above. Mike Ash provides a terrific introduction to ARC along with some pitfalls.
One of the pitfalls of using ARC is that weak references (aka zeroing weak references) are not supported on iOS 4 or on Snow Leopard. You could use Mike’s MKZeroingWeakRef class with all weak references, but now there is a better option: PLWeakCompatibility. This package allows you to use all the goodness of ARC weak references with the native syntax. Brilliant!
You can even get PLWeakCompatibility on GitHub so you can keep up to date on its progress.