In 8.1i, PACE will not be invoked for the Assign Package Pins process if the UCF file is selected.
If the EDIF or NGC/NGO top-level source is selected instead, the Assign Package Pins process works correctly.
This issue is fixed in the latest 8.1i Service Pack, available at:
http://www.xilinx.com/xlnx/xil_sw_updates_home.jspThe first service pack containing the fix is 8.1i Service Pack 1.