EXACT same problem!
Here is a copy of the email I just sent Da-Lite,
"I have a 57x77 Model B High Power screen that has a problem. There is a dark vertical band that is approx. 6 inches in diameter that runs the entire height of the screen. This band is located about 1-2 feet away from the left side of the screen. It is most easily seen during dark scenes. The screen was purchased from Cousin's Video in Mentor OH on 10-26-00. Thank you for your assistance with this matter."
System info: Native XGA LCD FP, Dalite 57X77 HP, P3 700, ATI Radeon, Cinemaster 2k, WinDVD 2.6.4, WinME