It's a jailbreak app on Cydia called "Firmware 3.1.3". It changes a value in a .plist file (I believe) to trick apps into thinking that you're running 3.1.3.
Evernote is catching flack from some of us on their forums for requiring 3.1.3 to install their universal binary (iPhone + iPad). The installation process, if done from iTunes, wipes out the existing copy of Evernote on the device and THEN alerts you that you don't have the proper OS version. If you don't have a backup of the previous version of Evernote, you're out of luck. Evernote's response so far has been "upgrade", while other developers have proven that 3.1.3 is not necessary to create a universal binary. Dragon Dictation initially released their binary and required 3.1.3, and then the very next day, released an update that removed that restriction. That's how you respond to your customers.