If you are running the free version of ESXi (aka ESXi Hypervisor) then you'll want to be aware of a critical issue that surfaces after upgrading to 5.0 Update 1. I want to stress this only applies to people runing the free ESXi version. If you're ESXi hosts are licensed this issue does not affect you.

There were some changes made in the way the ESXi APIs are accessed in Update 1 that unfortunately breaks the VM Auto Start feature in the free ESXi version. Please note that this issue only affects the free ESXi version and it was not intentional. I've heard some people making claims that VMware did this on purpose as a way to further limit the free ESXi version, this is not true. I'm not aware of a workaround, but many customers have been able to leverage the dual-image architecture of ESXi in order to roll-back to the pre-upgrade state.    

--> Pleasse see the rest on the blog : http://blogs.vmware.com/vsphere/2012/03/free-esxi-hypervisor-auto-start-breaks-with-50-update-1.html