There's a thread a while ago about the Roku HD1k having a MAC address of all 0's after doing a factory reset, and of course it won't talk on the LAN with an all 0 MAC address. A employee of Roku had a message in that thread to PM him for the solution. I did, and he said he doesn't work for Roku any longer, but still gave me the instructions and said I should post them here if they work, and they do.
What you need:
Empty CF card
Serial cable (straight through, pin2 to pin2, pin3 to pin3)
Place one file on the empty CF card, autoexec.rok, and inside that file, put the contents "exit" (without the quotes) and a new line (press enter). Connect the serial cable to a computer, and run a terminal emulator (hyperterminal in windows, minicom in linux) with settings 115200-8-N-1 (no flow control). Insert the CF card into the roku, and reboot it (by holding the power button until the light goes off). After a bunch of debugging messages, you should get the MMON> prompt. At that prompt, type in
pg8100 raw 00:0d:4b:XX:YY:ZZ
where XX:YY:ZZ is any 3 bytes of your choosing. Remove the CF card and reboot again. Your MAC address will be set to whatever you typed above, and your HD1K will talk to the LAN again.