After a complete SAN/ESX power down today, the first since upgrading the ESX hosts to ESXi 4 I came across the 503 Service Unavailable message when trying to connect to the Web Console to power up the host machines. It turns out that since the upgrade to ESXi 4 the Web Service is not started by default. Which of course presents a bit of a problem when your DNS servers are virtual machines on your VMWare hosts.
To resolve this you first need to gain access to the console by pressing ALT+F1 then run the following command to confirm the Web Service is not running:
service vmware-webAccess status
If the service is not running then issue the command:
service vmware-webAccess start
Please note the capital A in the command as it is case sensitive, this will then allow you to use the web interface as normal