Yesterday, shortly after I wrote the Xbox 360 HD DVD drive article, my local store got the Xbox 360 HD DVD drives in, so I was able to try it myself on my Macs. As expected, the drive is recognized immediately.
There is a separate Xbox 360 HD DVD Memory Unit listed because HD DVD players must include non-volatile memory for things such as user bookmarks, etc. On my Macs CDs and DVDs play fine using this drive, but commercial HD DVDs are not natively supported by OS X Tiger (yet).
With Windows Vista on my MacBook, commercial HD DVDs are recognized (but are not playable without the correct software). The drive is recognized as a Toshiba DVD/HD X807616 USB Device. Out of interest's sake, I checked out the disc sizes of some HD DVDs. King Kong (a 188 minute movie) is 27.4 GB, but Corpse Bride is only 19 GB. The Polar Express, a reference quality 100 minute disc, is reported to be only 13 GB.