We have just bought a new Virtex4 based devleopment board, but were having alot of trouble with the on-chip block RAM. This same design run successfully on a Virtex2 board. We were running ISE 6.3.
We just got ISE 7.1 and our problems have gone away. Just a warning to any who might be having problems with Virtex4 block RAMs and are using ISE 6.3, upgrade to ISE 7.1 ASAP.