I am using a 25ft DVI cable for my HDTV. Got it a circuitcity for around $100 (Monster Cable). Real pricey and probably overkill but I have not noticed any signal loss or poor quality due to the cable length. I am also using 8 25f+ VGA cables on a multiple-monitor workstation with the same flawless results. No repeaters or anything like that are being used. All nVidia video cards (AGP, PCI).
So, you shouldn't have any problems using a DVI cable longer than 6ft. I used Cables2Go for my VGA cables. Decnet company.