ISE 12.2 is out. It seems "LIT:600 - IOBUFDS symbol" problem was addressed in 12.2. With ISE 12.1, we often got
"LIT:600 - IOBUFDS symbol ... does not have IOSTANDARD specified. Map is unable to generate a default IOSTANDARD for IOBUFDS, one has to be explicitly provided."
which is similar to the one in the Xilinx forum.
With ISE 11.5, the above problem was not there.