I've been having the same issue for months now across several 5.1 and 5.1u1 systems. They include R510, R710, and R720 servers. They all have internal PERC controllers with direct attached storage. I found a workaround at the Dell TechCenter forums:
http://en.community.dell.com/techcenter/systems-management/f/4494/p/19491101/20322503.aspx#20322503
SSH into your ESXi host and run:
/sbin/services.sh restart
This allowed me to use OpenManage Server Administrator across all of the hosts.