Using A3.1i_sp5, I am targeting a Virtex-E design. When I attempt to select the LVCMOS18 I/O standard in Constraints Editor, the tool allows me to select it in the drop-down menu, but I am unable to set it. As soon as I select the LVCMOS18, the box becomes grayed out and the I/O standard is not set.
To work around this problem, manually specify the I/O standard in the UCF file.
This was fixed in the 4.1i software.