VMware have just released a new KB article 1003490 which focuses on restarting the Management agents on both ESX and ESXi Hosts from Version 3 through to Version 4.

Restarting the management agents on Hosts is often used when you are unable to connect your ESX/ESXi Hosts to vCenter or you can’t connect directly to your Hosts using the VMware Infrastructure Client.

The KB Article explains other symptoms which can be resolved by restarting the agents, it also be contains two video’s, one for ESX and one for ESXi (Shown below)

Restarting the Management agents on ESX

Restarting the Management agents on ESXi

Both video’s and other handy articles can be found on the VMware Self-Service Knowledgebase