For ESXi 5.X see http://kb.vmware.com/selfservice/search.do?cmd=displayKC&docType=kc&docTypeID=DT_KB_1_1&externalId=1017530

Giving VMs in ESXi 3.5 direct access to a disk

Create the VM, create a blank virtal disk on the main datastore

Gain shell access via alt-f1, unsupported, root password method
If you want ssh access you need to edit /etc/inetd.conf, uncomment the line beginning with #ssh
then ps | grep inetd to find its PID; kill it with kill -HUP <PID>; inetd will auto-restart

cd to the vm's folder (for instance /vmfs/volumes/datastore1/my_vm)
find the disks's device mapping via fdisk -l | less
the main disk will have about 8 partitons on it, don't use that one
then run
	vmkfstools -i myvm.vmdk -d rdm:/vmfs/devices/disks/vmhba0\:1\:0\:0 insert_name_here.vmdk
Where vmhba0\:1\:0\:0 is the device mapping you found with fdisk -l
Cancel (Ctrl-C) that command once it reaches 1% completion.
(BTW, rdm stands for raw device mapping)

Now you should be able to go into the vsphere client and add the existing disk by browsing to the VM's folder.