Just been through this issue myself and wasted days on it. It's a Broadcom driver issue. There is a Broadcom bnx2 driver update, but if you install it in the wrong order you get a magenta screen of death (MSOD!?) on boot. Details on my blog here:
http://pcloadletter.co.uk/2012/01/17/upgrading-vsphere-5-0-with-dell-equallogic/