For developers who already have finished apps in the App store, when are you switching to building updates with the newer 2.1 SDK? And when will you start building app updates (or new apps) for iPhone OS 2.1 instead of building backward compatible app updates for OS 2.0? Has anyone heard any estimates with at least 50%+ of App store customers will be running 2.1? There's at least one 2.1 feature which I would like to add to one of my apps, but not if it makes an update unusable by the majority of its current users. .