according to http://nextmarvel.net/blog/2011/09/brew-install-postgresql-on-os-x-lion/ this script moves your OS X default postgres binaries into an archive folder and symlinks the homebrew versions in place of them

fixBrewLionPostgres.sh

#!/bin/sh

BREW_POSTGRES_DIR=`brew info postgres | awk '{print $1"/bin"}' | grep "/postgresql/"`
LION_POSTGRES_DIR=`which postgres | xargs dirname`
LION_PSQL_DIR=`which psql | xargs dirname`

sudo mkdir -p $LION_POSTGRES_DIR/archive
sudo mkdir -p $LION_PSQL_DIR/archive

for i in `ls $BREW_POSTGRES_DIR`
do
  if [ -f $LION_POSTGRES_DIR/$i ] 
	then
		sudo mv $LION_POSTGRES_DIR/$i $LION_POSTGRES_DIR/archive/$i
		sudo ln -s $BREW_POSTGRES_DIR/$i $LION_POSTGRES_DIR/$i
	fi
	
	if [ -f $LION_PSQL_DIR/$i ] 
	then
		sudo mv $LION_PSQL_DIR/$i $LION_PSQL_DIR/archive/$i
		sudo ln -s $BREW_POSTGRES_DIR/$i $LION_PSQL_DIR/$i
	fi	
done