The Spartan-3 default VCCAUX value appears as 3.3 V in the XPower interface as well as in the report file. The VCCAUX should be 2.5 V.
This is an error. You can edit the VCCAUX field and change it to 2.5V.
This problem has been fixed in the latest 6.2i Service Pack, available at:
The first service pack containing the fix is 6.2i Service Pack 1.