After installing VMWare 5.0 and doing an update to 5.1 I got this error.

No vmkcore disk partition is available and no network core dump server has been configured. Host core dumps cannot be saved.

ESXi basically wants to know where to dump the OS in an event of Purple Screen of Death.

After some digging around I found this KB. KB2001566
This didn't help much if you want dump to a LUN.
To configure the dump to LUN look at this KB2004299

Basically do install esxcli from vmware.com

esxcli -s "servername" system coredump partition get

Active:
Configured:

Came back blank as its not configured.

List the drives.

esxcli -s "servername" system coredump partition list

The output appears similar to:

Name Path Active Configured
--------------------------- -------------------------------------------------- ------ ----------
naa.60060160028131:7 /vmfs/devices/disks/naa.60060160028131:7 true true

Once you have a drive set it to be to core dump.

esxcli -s "servername" system coredump partition set --partition="naa.60060160028131:7"

esxcli -s "servername" system coredump partition set --enable true

Check to make sure the drive is now set.

esxcli -s "servername" system coredump partition list

The output appears similar to:

Name Path Active Configured
--------------------- ------------------ ------ ----------
naa.60060160028131:7 /vmfs/devices/.... true true