It seems that several people have been having problems getting Dell OMSA 6.2 to work correctly on CentOS 5.4 x86_64. Specifically, the software does not detect any storage controllers, and therefore also doesn't find any disks. eg.
[root@b034 ~]# omreport storage pdisk controller=0 Invalid controller value. Read, controller=0 No controllers found.
After a little investigation, I found the source of the problem.