You obviously don't even know what HDCP is or you would know it has absolutely no bearing on SPDIF for audio output.
Since HDCP is required (by the studios and the licensing bodies) in order to play HD-DVD and BLU-RAY over digital video connections not supporting it would have eliminated the PC as a platform for those formats. That is why MSFT chose to support it in Vista.
DRM-wise Vista and XP are pretty much the same: they support it. Don't like it? Then don't buy DRMed media. It's that simple.
By that I naturally meant that if Vista is a requirement to get HDCP support and through that the S/PDIF will get disabled (which btw is directly related to the HDCP support as RIAA/MPAA require every non-drm output to be disabled), it's not even an option. But for some reason you seemed to feel the need to try to twist my post for your own need.