In my design with a differential output registered inside of the IOB, the OFFSET/OUT constraint is analyzing incorrect paths. The longest path on the "n" side is shown to be from the reset flop through the "n" side data FF to the output pad (reg_sr_q is not enabled). This path is not reported for the "p" side, and should not be reported for the "n" side.
This issue affects all Virtex-II families (Virtex-II, Virtex-II Pro, and Spartan-3). The tools have been modified to contain a new timing parameter named "Tiosrdo" to describe these paths and allow TRCE/Timing Analyzer to filter out these specialized paths. The first release containing this fix is 6.2i, scheduled for February 2004.