I am in the same boat; I have an Apple 23" with ADC to DVI adapter, only I am
running a GeForce 6600. I have seen many posts from folks using nVIDIA
display cards that can't get DVI to work at all. VGA works fine, but not DVI.
There's even some people trying to use HDMI and hook up their plasma displays
to no avail. My only monitor is the digital Apple display, so I can't even
use Windows Vista Beta 2 until this issue gets fixed. Right now digital
output (DVI) from nVIDIA cards is seriously broken for Vista. We're gonna
have to sit tight until an update comes. It's a bummer.