A problem has been found with the Virtex-4 DCM Auto-Calibration Macro for stepping 2 level devices.
This problem is fixed in ISE version 10.1. Meanwhile, the following data file patch is compatible with all platforms and all ISE revisions from 8.2i and 9.2i:
To install, unzip in the Xilinx install directory while maintaining directory structure.
See (Xilinx Answer 21435) for more information regarding DCM Auto-Calibration.