This is a 5.1 version based internal firmware. It is known to run on iPhone 3Gs, iPhone 4 and iPod Touch 4G. Due to the fact that it contains kernels for other devices (something which is common for most SwitchBoard filesystems) it can possibly be installed onto other devices.
This firmware was leaked most likely as a root filesystem dump.IPSWs for iPhone 3Gs, iPhone 4 and iPod Touch 4G have since appeared and can be restored to these devices via bootrom exploits.
This build contains the following applications applications launched through SwitchBoard.app: