From ESX3.5i to vSphere VMWare has changed slightly the location of where to configure your host so that the guests restart if you happen to have either power failures to your ESX host or if you just shut it down etc..
So follow the below steps to ensure your guests power on automatically on host startup.
Open vSphere Client
In the left hand pane, highlight your choosen host, then click the configuration tab in the main pane.
Click Virtual Machine Startup/Shutdown under the software Section.
Click Properties in the top right hand corner of the main pane.
Then check Allow virtual machines to start and stop automatically with the system, then change the startup order of your guests in the box below as appropriate.
If you want all to startup with the host, then move them all up to automatic startup.
Now when your host restarts, your guests will come up automatically as well.