This problem is scheduled to be fixed in ISE version 11.1. Meanwhile, if Physical Synthesis options are used, it is advised that the resulting implementation be analyzed for unconstrained paths.
The design exhibiting this issue in ISE version 9.1i did not exhibit the issue in ISE 10.1, and it was also noted that no SRL transformations took place. ISE 10.1 is less likely to perform SRL transformations than 9.1i because of a change that eliminated some suboptimal transformations from occurring. For that reason, timing paths are less likely to be affected in 10.1 than in 9.1i.