Xcode 4.2 with ARC: will my code run even on iOS devices with firmware older than 5.0?

I updated my Xcode to 4.2 version, whi开发者_如何转开发ch includes the ARC technology. It seems to be a good thing, but if I enable ARC and edit my code according to Apple's suggestions, will my app build&run even on 4.3.x devices? Or only for iOS 5 ones?

ARC applications will run on OS 4.0 (4.3?) and up. On 4.3 you will lose the zeroing-weak-reference feature though. So if you rely on __weak, you shouldn't expect to work properly on < 5.0.