Guys - I was hoping to help but may have some contradictory / conflicting info.
There's a statement in this month's DVD Review mag that says categorically that the HD-DVD medium is region free. I have the 360 HD drive and have had no problems with imported discs so far (V for Vendetta, Children of Men and Serenity) so based on that I'd have said you should be fine.
However, when you look on playusa.com at the HD-DVDs there is a statement that they will only play on American players. I'm not 100% how accurate that is or whether it's there to deter people but on discs such as Children of Men which has the HD on one side and SD on the other, the SD side is region coded as Region 1.
Also, the last time I went to a fair, one of the traders was implying that region coding was likely to be introduced on HD at some point but to his knowledge they're region free at the moment.
Sorry I couldn't give you a definitive but would rather at least provide some relevant info.