Well, except for a few (useless/DRM-crippled) high-definition WMV-HD discs out there, you can't buy many high definition discs. The only way to experience HD movies is to either catch them on an HD movie channel (where they upscale it for you), or upscale it yourself with your DVD player or TV.
If you want to watch native HD content, you'll have to wait until the new Blue-Ray Discs or HD-DVD's come out at the end of the year.