It's not the processor that's the issue, it's the blade configuration that's the issue. I'm runing my host (with ESXi 5 installed) with a pair of E5405 Xeon's without issue. You need to ensure the actual blade (and possibly chassis too) is on the HCL/compatible with ESXi 5. From what I could find on the VMware HCL, the family of blades you're using only went up to ESX/ESXi 4.1 u2.
BTW, not terribly surprised by the lack of help from IBM here.
