PAL and NTSC are non-issues when talking about high def. What may be an issue is the 50Hz (Europe) or 60Hz (US) and the framerate, i.e. 24 or 25fps, that traditionally divides our releases. However at least one studio seems set on releasing their first UK discs with US specifications which would suggest they will play on US machines. Still we won't know until we see it.
Region coding may also be an issue in the future. Currently no region coding exists on HD DVD but it is being discussed. If it is implemented nobody knows yet whether it will be crackable or whether the players available now will ever be subject to it.
In summary alot of unknowns that will remain unknown until (probably) September when we'll get a hands on with the UK player at the Berlin festival. What I think is fairly certain though is that for the time being the bulk of quality HD DVDs will be US in origin...