I was trying to do an install of ESXi 5.1 U1 on a Cisco C240 M4 and getting a really pretty purple screen saying: NOT_IMPLEMENTED bora/vmkernal/hardware/apic.c:1048
After a frustrating call to VMware support, I threw my hands up and went to the web to fix it myself (I Googled it of course).
I got what pointed me the right direction from this link.
Steve_Delassus pointed at a KB article and then said something simple...."Check the compatibility matrix and see if your server is in it".
I kicked my self a couple of times, then downloaded 5.1 U3 and it installed with no issues....
So what have I learned Boys and Girls? The VMware Compatibility Guide is your friend!