A customer has found a problem where his TIGs are not being used. He has TIGs on three nets. And when Trace is run, those three nets show up in the paths covered by timespecs. However, when the customer introduced a dummy path that is just an ipad->ibuf->obuf->opad, and placed a TIG on the net going to the OBUF, the other three TIGs work. When he runs trace, the paths covered by timespecs are now correct.
A fix for this problem is included in the 1.5i Service Pack 2. For details
on this Service Pack see http://www.xilinx.com/techdocs/5887.htm