Had a question about this...
Are you sure?
I can't find any documentation at all from Cisco that indicates exactly how this port can be used.
Since the home is provided Ethernet, phone, and cable TV all over the fiber connection to the home, which then carries it the last bit throughout your home on a piece of coax, then all it would take would be a decent chip to pull the home network ethernet connection off the coax and make it an available internet feed from the network.
It's struck me as one of the best possible features for these boxes! The coax is already at the TV location, the TV and Blu-ray player, and receiver all need Internet connections, so the cable feed provides the STB with Internet, and the LAN port on the back acts as a Ethernet output port. Like a feed off a switch.
I have a customer who has several RNG 150 boxes as well as some other boxes and I wanted to actually test these connections to see what I would get, but because there is zero documenation that I have found, I wanted to know if you had found documentation or are 100% sure that the Ethernet jack does not possibly offer a connection to the Internet.
It would make a lot of sense for the STB to not have a router, but to have a network switch internally. If it is fed coax (w/Internet connection), the STB is given an IP address on the home network and is visible to the cable company (it is for sure). So, it then only needs to provide an output feed which also allows for Internet capability. I certainly would have designed the STB this way. But, they tend to be pretty poorly design, so who's to say?
edit: Looks like it may work, but with a note of caution (of course)...
http://www.dslreports.com/forum/r24537696-UVerse-IPN4320-network-connection