As Clare quite correctly states, HD-DVD is not currently region coded but can be in the future if they wish. The players ARE coded for region 1 - 6 and , A, B and C for DVD and HD-DVD respectively but the HD-DVD discs are not.
With the current problems associated with Blu-Ray, I think this lack of regional coding, and the finalisation of the format is the biggest guns that the format has in the war with Blu-Ray and could be decisive.
The different regional versions of the same discs mentioned by Clare refers to content and I believe is pretty unavoidable due to nationally differing censorship regulations but there is no coding currently preventing an unrated US "A" disc playing on a UK "B" player.
Gerry.