A lot depends on the equipment and the scaler used by the dvd player or the built in line doubler of the television/projector.
I can definately tell a difference between 480i to 480p, and also from 480p to 720p or 1080i on my Toshiba 32" or my Sanyo Z2 projector.
An interlaced signal is created by making two passes on the screen. First the odd numbers 1, 3, 5....479 top to bottom. Once this is complete the even numbers 2, 4, ....480. This process takes 1/60th of a second per pass or 1/30th second total time to complete.
The progressive process scans all lines sequentially 1,2,3,....480 in one pass. This process reduces jaggies and other artifacts that come from an interlaced process. In addition all the lines are scanned in 1/60th of a second in one pass.
Anyway, my experience is that there is a difference in 480i versus 480p. Your mileage may vary.
