You might be able to "force" a standard dvd to play at 1080, but it wont have the clarity of a true 1080 (Hi Def / Blue Ray). The blue ray does hold more the data, but all that extra space is needed to hold the massive amounts of extra data of a true HD dvd. I dont have a high def TV, but I know my digitial cable box is set to 1080 output. Even though my cable box is set to 1080, my TV in no way could possibly show the true clarity of a Hi Def TV that truely supports 1080.
I was at a friends house watching a blue ray, and someone sneezed and you can literally see thousands of tiny spray particles , where as on my TV, it just a quick flash of blur.