To work around this issue, open the data sheet from the main CORE Generator GUI. The problem only occurs when you open a core customization GUI and then try to click on the View Data Sheet button.
This problem has been fixed in the latest 9.2i Service Pack available at:
http://www.xilinx.com/xlnx/xil_sw_updates_home.jspThe first service pack containing the fix is 9.2i Service Pack 1.