Apple has issued a fix for iOS 3.1.3 App Store download bug. The bug in question removed the 'Download' button altogether when users running the older firmware attempted to acquire apps from the App Store.
Since the App Store is essentially a native wrapper around a web-based interface, Apple is able to update it without having to push out a full version of the whole OS. That can lead to things breaking out of nowhere, but also getting fixed.
It's good to know that Apple still has the backs of those running on firmware that's now going on two years old.