It depends on the equipment (TV) you are using. If you use a DLP or simular tv (Digital) it will improve the picture , but if you have a CRT based tv you will not really notice any change because there has to be a analog to digital conversion in the chain somewhere. DVI is not really intended on improving Picture Quality it is designed mostly for copywrite protection.
For my setup, 811 > DLP projector, I have both hooked up also. With the DVI cable I see a picture that looks like the 'sharpness' is turned up. Grainy is not the word I want to use. Looks smoother with the component.
I still haven't figured out which I like better. For the SuperBowl I ended up watching with the component. But for Discovery HD Theater programming, scenery etc, I have been using the DVI.
Before the projector I had a Toshiba 65" RPTV and I didn't notice any difference between the DVI and component.
Thanks for all the responces. My TV is a Rear Projection Mitsubishi 55". I will go and get a cheap cable and try it out even though it sounds like it will not do much since it is a Rear Projection. Thanks again..