#!/bin/bash
echo "Set X credentials in the sudo session to user: "$1
su - $1 -c 'xauth list' |\
     grep `echo $DISPLAY |\
         cut -d ':' -f 2 |\
         cut -d '.' -f 1 |\
         sed -e s/^/:/`  |\
     xargs -n 3 xauth add

Tribute to http://joelinoff.com/blog/?p=729