The PIN2UCF utility is installed in the same location as the Xilinx tools. You can run this utility on the command line or in Project Navigator. When this utility is run, a UCF with the pin assignments from a previous run is created.
You can use the PAD2UCF Perl script available at:
This script processes the design.pad report produced by PAR and recreates the equivalent UCF constraints that would have locked the signals to the desired pads. You can then use this UCF for subsequent PAR runs.
NOTE: This script assumes the name of the signal in the UCF is the name of the IOB reported in the design.pad file. This assumption should always be true, provided that a BLKNM or HBLKNM was not placed on the pad.