Get ssh-copy-id in Mac OS X http://www.devthought.com/2009/09/19/get-ssh-copy-id-in-mac-os-x/

ssh-copy-id

#!/bin/sh

# Shell script to install your public key on a remote machine
# Takes the remote machine name as an argument.
# Obviously, the remote machine must accept password authentication,
# or one of the other keys in your ssh-agent, for this to work.
#
# http://www.devthought.com/2009/09/19/get-ssh-copy-id-in-mac-os-x/
#

ID_FILE="${HOME}/.ssh/id_rsa.pub"

if [ "-i" = "$1" ]; then
  shift
  # check if we have 2 parameters left, if so the first is the new ID file
  if [ -n "$2" ]; then
    if expr "$1" : ".*\.pub" > /dev/null ; then
      ID_FILE="$1"
    else
      ID_FILE="$1.pub"
    fi
    shift         # and this should leave $1 as the target name
  fi
else
  if [ x$SSH_AUTH_SOCK != x ] && ssh-add -L >/dev/null 2>&1; then
    GET_ID="$GET_ID ssh-add -L"
  fi
fi

if [ -z "`eval $GET_ID`" ] && [ -r "${ID_FILE}" ] ; then
  GET_ID="cat ${ID_FILE}"
fi

if [ -z "`eval $GET_ID`" ]; then
  echo "$0: ERROR: No identities found" >&2
  exit 1
fi

if [ "$#" -lt 1 ] || [ "$1" = "-h" ] || [ "$1" = "--help" ]; then
  echo "Usage: $0 [-i [identity_file]] [user@]machine" >&2
  exit 1
fi

# strip any trailing colon
host=`echo $1 | sed 's/:$//'`

{ eval "$GET_ID" ; } | ssh $host "umask 077; test -d ~/.ssh || mkdir ~/.ssh ; cat >> ~/.ssh/authorized_keys" || exit 1

cat <<EOF
Now try logging into the machine, with "ssh '$host'", and check in:

  ~/.ssh/authorized_keys

to make sure we haven't added extra keys that you weren't expecting.

EOF

brew install ssh-copy-id
or
port install openssh

This will fail with an "Ambiguous Output" error for hosts where the shell is not defaulting to bash. For instance, csh.
See here for more on the problem and the fix

{ eval "$GET_ID" ; } | ssh $host "exec sh -c 'umask 077; test -d ~/.ssh || mkdir ~/.ssh ; cat >> .ssh/authorized_keys'"

ssh $host "exec sh -c 'umask 077; test -d ~/.ssh || mkdir ~/.ssh ; cat >> .ssh/authorized_keys'"