Are storyboards going to work on ios 4?

Storyboards are new features in xcode 4.2 which simplify navigation design.
I assume new apps using storyboards are going to be working just fine on iOS 5 when it ships but I haven't seen anything about storyboards compatibility with iOS 4.
It makes sense that they would work (just like ARC is supposed to be going to work - they are a compiler / IDE feature, not runtime features AFAIK) but that's just me guessing, does anyone know if it's safe to use storyboards and be able to ship for iOS 4?

thanks!