This is known to work in Splunk 5+. Please refer the attached image
Both in the server.conf and web.conf for the trustedIP property you need to set
trustedIP=::1 NOT trustedIP=[::1] , if you do you would likely see following error in the splunkd.log
-04-2014 21:58:31.362 -0800 ERROR HTTPRestLogin - SSO failed - Given IP '::1' does not match trusted IP '[::1]'