Very cool feature which you can enable by using the Viclient.

Go to

Configuration,  
Advanced Settings,  
VMKernel,  
Boot,  
VMkernel.Boot.serialPort

There you can set your wanted Serial Port.
0 means “no Output”, 1 “Com1”, 2 “Com2”, etc…

Just enter you wanted Serial Port, Ok, Close, Shutdown the Server, hook up to the Serial Port with an Console Cable, get your Console open (115200er Speed, you can change that aswell!) and boot up the Server… Tada! Ok, thats only the log for Bootup!

If you want an Output for “Working ESXi”, you need to go to

Configuration,  
Advanced Settings,  
Misc,  
Misc.SerialPort

Settings are the same as above.

Enjoy