The OFFSET IN constraint using the new DCM Rising/Falling keywords ignores DCM phase. The DCM phase appears in the PERIOD constraint, but the OFFSET reports "Rising at 0."
This problem has been fixed in the latest 10.1 Service Pack available at:
http://www.xilinx.com/support/download/index.htm
The first service pack containing the fix is 10.1 Service Pack 1.