Doing a TIG in the .UCF file on a signal or signals which is the output of a tristate buffer does not work. The TIG is ignored and the signal will show up in the .TWR report. The only workaround is to put each signal that you wanted ignored, into the .PCF file, which will work in not doing any timing analysis on the specified paths.
A patch is available on the FTP site:
This patch is also available from the M1.3 Performance Pack CD.