In ModelSim, 6.6c X's are seen in functional or timing simulation, and simulation fails to complete successfully.
There is a known issue in Modelsim 6.6c that is scheduled to be fixed in Modelsim 6.6e.
This issue can be worked around in Modelsim 6.6c when either all the signals are dumped in the waveform (add list -r *), or when all signals are logged by the simulator without displaying any extra signals in the waveform window (add log -r *).