I have a Philips Roku TV that I purchased from Wal-Mart for about 125 dollars. It does not have an ethernet port, but it does have a have USB port.
The WiFi signal in my garage is poor and I decided to run an ethernet port to the TV. My biggest concern was getting the correct usb ethernet adapter to work with my TV.
After reading this post on the Roku forums about different TVs/USB adapters working together, I decided on the Plugable USB 2.0 ethernet Adapter 10/100 Mbps.
After plugging everything up, you will need to reboot the the TV for the usb Adapter to start working, and it will take a while as I can only guess that the TV’s OS (linux?) is installing the drivers for the usb ethernet adapter. Once it finished booting up the ethernet port was working like it should.