On a DELL R610 system (dell-per610-01.lab.bos.redhat.com for example) with Nehalem-EP CPU's running RHEL-5.6 EDAC does not recognise the memory controller.

It loads the module correctly:
root@dell-per610-01 ~]# dmesg |grep -i EDAC
EDAC MC: Ver: 2.0.1 Feb 18 2011
EDAC i7core: Driver loaded.

And you can see the mc in the /proc interface:
[root@dell-per610-01 ~]# ls /sys/devices/system/edac/mc
log_ce  log_ue  panic_on_ue  poll_msec

But it's not actually seeing any Memory Controller, as normally you'd expect something like this:
EDAC MC0: Giving out device to i7core_edac.c i7 core #0: DEV 0000:ff:03.0
EDAC MC1: Giving out device to i7core_edac.c i7 core #1: DEV 0000:fe:03.0
which is not the case here.
 This is corroborated by the output from edac-util -r:
[root@dell-per610-01 ~]# /usr/bin/edac-util -r
edac-util: Error: No memory controller data found.