HD-DVD and Blu-Ray have different numerical aperture (0.65 and 0.85 respectively).
There are other differences however, namely in channel coding, rotational speeds, pitch, read-power and several margins. And that doesn't even touch on the media format issues (or logical formats).
I'm not an expert on the issue, but I haven't even seen an optical pick-up head that supports both formats.
More than likely it's possible to make one, but at this stage of the format battle, I can't see either camp being really eager to support such an effort (?).
However, if neither of the formats really fall on their face and die away quickly, then I can't see any other possibility except there being universal players at some time in the future.
It took a long time for dvd-a/sacd players as well, so it could take a long time for hd-dvd/blu-ray recorders/players.