If you still have iOS 3.1.3, you can not download applications from the iPhone

According to some users are reporting on Apple's official forums, from the December 16 IOS devices (iPhone and iPod Touch) that still have version 3.1.3 installed, you can not download applications directly from the application from the App Store.

The process followed in this case the application is purchased through iTunes and sync the device normally.

Apple has not commented about it but you can check yourself as on the official forums and Engadget have echoed the news. Another problem for all iPhone 3G users who decided not to upgrade to IOS 4 for performance issues.

Source: Engadget