A few weeks ago, there were some reports that the Xbox 360's HD-DVD drive could be plugged into a PC and was recognized as an optical drive. Robert McLaws of Windows-Now has been in touch with Microsoft and has learned that this is true. However, you will need to buy additional software if you plan to play back HD-DVD movies.
A member of the Xbox 360 HD-DVD team confirmed for me today that the Xbox 360 HD-DVD drive will in fact be recognized by Windows Vista, but it will NOT ship with decoder software. I'm assuming that Intervideo will have a version of WinDVD sometime in the future that supports HD-DVD. Personally, I think the drive should come with the required software to dual-use enable the device (adding value to a non-trivial purchase), but I'm not a PM at Microsoft.
Hopefully, InterVideo or CyberLink will release software that is compatible with the drive. If you'd like to read more, the entire report can be found here.