Here is a slick way using host profiles. It calls for using a host profile and using the Configure a fixed administrator password option. Details at:
http://www.vladan.fr/how-to-reset-esxi-5-x-root-password-and-under-which-conditions/
But, it may not be supported. The statement in the following KB article indicates that the only supported way to reset the root account password on an ESXi host is to reinstall ESXi.
