In the 5.1i software with Service Pack 2, DRC flags an error for a design containing 512x36 Block RAM that is used with a MULT18x18. The error message is similar to the following:
"ERROR:DesignRules:620 - Blockcheck: Illegal RAMB16 and MULT18X18 placement. The comp mq1 cannot be placed next to 512x36 Block RAM comp r2. There are not enough routing resources."
You may safely ignore this DRC error message if it is the only error message you receive (i.e., no other error messages report not being able to fully route/place the design).
The bitstream can be safely generated by disabling the DRC option as follows:
bitgen -d infile.ncd outfile pcf_file