The following devices are not in the 9.1.01 or previous versions of the ISE design tools:
xa4vfx12
xa4vfx12_ffg668
xa4vfx12_sfg363
This problem has been fixed in the latest 9.1i Service Pack available at:
http://www.xilinx.com/xlnx/xil_sw_updates_home.jsp
The first service pack containing the fix is 9.1i Service Pack 2.