All Spartan 3 families infer the IDDR logic for RGMII. The recent addition of locally synchronized resets has prevented this inference from taking place.
The rgmii_v2_0_if file has been updated in the Tri-Mode Ethernet MACv4.3 rev1 patch to use the same reset for both the rising and falling edge registers to ensure they are correctly mapped into the IOB IDDRs.
To get the patch see:
(Xilinx Answer 33307) LogiCORE IP Tri-Mode Ethernet MAC v4.3 - Release Notes and Known Issues for ISE 11.3