While fragmentation is a rare(r) problem on iOS
there's still quite a few users rocking older versions on their devices. Unfortunately for them, ever since a recent update on the 16th they haven't been able to download any new apps from the App Store straight to their devices. While users of older iPod Touches and iPhones stuck on 3.1.3
by choice or incompatibility with newer OSes can still update the apps they already have and transfer software from iTunes on a computer, they can't buy or install new ones directly from the device. We were able to confirm the issue on one of our own devices, while there's a pretty lengthy thread in Apple's support forum about the issue, but no official response yet. We'll keep you posted if word comes that this is just a glitch or if the company has decided to leave those old versions on the outside looking in.