7.1i PAR does not allow the GT11 input clocks to be driven by BUFRs. These include USRCLKs.
This issue is under investigation, and whether this use model will be supported in the design tools will be resolved by ISE 8.2i.
GT11 input clocks can be driven by BUFGs.
There is no restriction on GT11 clock outputs driving either a BUFR or a BUFG.