John Porcaro has reiterated  over at the Gamerscore blog that there are no plans in the works to release game titles on HD-DVD using the upcoming add-on drive. 

Since announcing the Xbox 360 HD DVD Player accessory at E3 2006, we’ve been clear that it is designed exclusively for playing HD DVD movies.  It will not play games on HD DVD.

There you have it.  Time to move along now back to debating the cost of the HD-DVD player. ;)