When I move or resize a slice-based area_group, if it is placed on an invalid location such as a bram column/central IOB column the area_group will disappear.
This problem has been fixed in the latest 8.2i Service Pack available at:
<http://www.xilinx.com/xlnx/xil_sw_updates_home.jsp>
The first service pack containing the fix is 8.2i Service Pack 1.