The iMPACT GUI should always open up unless "Automatically Generate PROM or ACE File" is "on". (If this property is checked "on", iMPACT should run in batch mode with the command file.) Deleting the path to the IPF file from the "iMPACT Project File" property allows the iMPACT GUI to launch.
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.