Android 4.0.4 has leaked for the Sprint Nexus S 4G, and is ready to flash with Odin. It bumps the system up to IMM76D, like the rest of the devices running 4.0.4, and contains a new set of images as well as a new radio. We've seen Ice Cream Sandwich leak before for the Nexus S 4G, and this ROM looks to be pretty much the same type of case.
It's officially signed off by Google with release keys, but that doesn't mean anything for the radio and Sprint specific network code inside the ROM itself. You'll be flashing this one at your own risk -- but should be able to roll back to stock in case of any issues easily enough. The folks in the forums are giving it a go, and early reports are looking good. Be sure to chime in with your own experience if you give it a shot. As for when we'll see anything official from Google and Sprint, nobody that knows is saying.