the current Vista SP1 RC - expiry date within 6 months - floats out there for feedfack fishing from microsoft.
there is just NO confirmed date anywhere, except statement like "slated for QT1 release ..."
take the 6 months rc testing, plus the marketing statement, u probably can get urself a pretty good release date
but there was also rumor that it will come out this February yet
not really a good answer to ur query, but that was my best shot