This is an EXCELLENT decision by Apple! It will not only help out the used market a little bit, but it also eases some of the pain of backwards-compatibility. Now when people can't or don't upgrade their devices, they aren't left in the dust - and they can at least use 'legacy' versions of Apps on iOS versions that don't support the latest and greatest.
I'm more excited about this than iOS 7. Well, almost.
I'm more excited about this than iOS 7. Well, almost.