Some hints for best playback of HD-DVDs with PowerDVD Ultra (HD) and reclock. This is the result from some experiments, it worked for me, maybe it works for others, too:
Start ReClock config, click the "Video settings" tab.
1.) Uncheck "determine frame rate of DVDs using built in estimator"
2.) Set "When framerate is not found... for DVDs" to 23.976
3.) Uncheck "Enable guessing a better media speed..."
Now start PowerDVD, play the movie. After the main movie has started, double click the left ReClock tray icon. Set the vide framerate to "Unknown". The icon will turn red.
With this method ReClock works perfectly even with movies which otherwise "resist" to be reclocked (for example Mission Impossible 3).
The vertical display refresh rate should be around a multiple of 24fps, of course (48/72Hz)