This squat little device is purportedly the new LG LX600 for Sprint, set to debut some time in the near future. An intrepid user on the SprintUsers.com forum found this image online, and it sure looks convincing to me. The LX600 is actually a clamshell that opens up to reveal a full QWERTY keyboard.
Like the Blitz, the LX600 is a "dumb" phone made just for messaging. It also has external music player keys for the built-in music player, as well as a camera, but there doesn't seem to be too much information about the other features. Engadget Mobile reports a rumored release on October 16, so I guess we have a while to wait to see if this is the real deal.