Finally, with ESXi 4.1 you can use AD to authenticate even if you are not using VCenter as a management source.
The key to use Active Directory Authentication with ESXi is this first step:
In Active Directory you must create a new group called “ESX Admins”. It must spelled exactly like that. Once you do this, then you will be able to add the ESXi server to the Domain using the Configuration, Authentication Services. Then after that, you add the users through the Permissions Tab.