VMware has published an article describing the method to install OS X 10.8 on ESXi 5.1. Previously, it is necessary to have updated all the installation with the latest version vSphere 5.1 and its ESXi in 5.1 too.
In brief, just choose Mac OS X 10.7 as guest operating system. OS X 10.8 is not integrated into the menu for the moment.