This is the post a lot of you have been waiting for.
The HD DVD add-on for the Xbox 360 works with a modded/hacked 360. It does NOT rewrite your firmware.
Here is the info:
Xbox 360 with Samsung-Toshiba drive (MS25) with Fall '06 update
Xtreme 4.2A firmware modded
HD DVD Player
I can watch HD DVD movies and play all my backups.
I believe the update disc just installs HD-DVD player software. The Xbox 360 recognizes the HD DVD player right when you plug it in. But it tells you that an update is required and please insert HD DVD update disc. The update is really fast.
Sorry, I don't know if the same is true for all those modded LG-Hitachi drives. Since Xtreme 4.2A has the Firmguard features, it may be that my firmware is protected from being overwritten. But I think the update is just to install some player software.
Hope this helps everyone,
P.S. HD DVD movies look pretty good and the load/response times are just as fas as my regular DVD player. So I guess this combo is better than the Toshiba stand alone players.