The best iPhone Apps are those that engage across other properties beyond the iPhone. Zynga, SGN, Playfish and others iPhone power-developers have done a brilliant job of those. For most though, building cross-platform, socially-aware applications has proven difficult and consequently made the majority of applications 'un-sticky'.
But this will all change with Facebook's announcement that Facebook Connect is available for iPhone Applications.
Suddenly gaming becomes social. Applications that before were content delivery mechanisms now have a social graph to relate to.
It's big. In a way it is akin to what Xbox Live does for Xbox... put probably bigger because it is open and more flexible.
InsideFacebook has some screenshots:
Now your iPhone apps can enjoy the benefits that Facebook Connect sites and Facebook Platform apps already enjoy, including:
Making API calls so your app can access users' profiles and share information on Facebook. Publishing to Facebook via Feed forms.
Asking users for extended permissions, like offline access, so you can still interact with their data when they're offline.