General Description: If internal signals are declared in a Little Endian-type format [0:N], XST will reverse the order to a Big Endian-type format [N:0]. This can cause problems with monitoring the proper signals when a back-annotated simulation is performed.
Solution
The only way to work around this problem is to name all of the signals in a Big Endian [N:0] format.
This problem has been fixed with the release of software version 5.1i.