An alternative explanation

Your conclusion that well-managed IT departments are quicker to adopt Vista because of the value it offers is not the only explanation for your data.

I would suggest that well-managed IT departments are more likely to tackle a challenging OS deployment because they are better equipped. Given equal motivation such organizations are more likely to schedule an earlier deployment than less well functioning departments.

Perhaps you should compare how much perceived value Vista provides to IT management in well-run and less-well-run organizations to validate your conclusion.