The DVD Player app can play MPEG-2 HD DVDs using the "classic" format (I've forgotten the proper name for it). Most commercial DVDs use the newer format with VC-1 video so DVD Player can't play them. In addition, the drive included with your system cannot read HD media.
It is possible to play HD DVDs using PowerDVD under Boot Camp, however you will need a drive too. I use an Xbox 360 drive, which now cost NZ$49 new so that's a cheap option if you happen to already own PowerDVD.
Warner is still making them, until the end of May. I find it rather ironic that the company credited with causing the downfall of the format is now the only one still making movies for it.