Depending on what version of the firmware you have presently, I know that there was at least one firmware update that I installed to allow a disc (one of the Planet Earth discs) to play properly. You can do the update off of a CD-R (get a friend to download it over broadband and burn it to a disc if you don't want to download it over dial-up). Try
this link for access to the firmware.
Here is some information on the update, and
here is the general HD-DVD firmware page.