Periodically scan your network searching for your Raspberry Pi board and update your /etc/hosts file with its ip address.

piping.sh

#!/bin/bash
#
# Periodically scan your network searching for your
# Raspberry Pi board and update your /etc/hosts file
# with its ip address.
#
# Copyleft by Simone 'evilsocket' Margaritelli
# http://www.evilsocket.net
# evilsocket at gmail dot com
#

# seconds to sleep time between one check and the next one
DELAY=5
# put your subnet here
SUBNET="192.168.1.0/24"
# put the first 2 octects of the Pi mac address
HWADDR="E8:94"
# hostname to put in /etc/hosts file
HOSTNAME="pi"

if [[ $EUID -ne 0 ]]; then
   echo "This script must be run as root" 1>&2
   exit 1
fi

while :
do
    echo "@ Searching for Raspberry Pi address ..."
    PIADDR=$(nmap -sP "$SUBNET" | awk "/^Nmap/{ip=\$NF}/$HWADDR/{print ip}")
    PIADDR=$(echo "$PIADDR" | sed -e "s/[^0-9\.]//g")
    if [ -z "$PIADDR" ]; then
        echo "! Could not find Raspberry Pi on network $SUBNET"
    else
        echo "@ Found '$PIADDR', updating /etc/hosts file ..."

        FOUND=$(cat /etc/hosts | grep -oE "\b([0-9]{1,3}\.){3}[0-9]{1,3}\b\s+$HOSTNAME\$")

        if ! [[ -z "$FOUND" ]] ; then
            echo "@ Removing old entry ..."
            cat /etc/hosts | grep -vw "$FOUND" > /etc/hosts.bak
            mv /etc/hosts.bak /etc/hosts
        fi

        echo "@ Updating /etc/hosts file ..."
        echo "$PIADDR $HOSTNAME" >> /etc/hosts
    fi

    sleep $DELAY

done

If more than a RPI are present in a network this script will slowly fill the /etc/hosts file.
example:

@ Searching for Raspberry Pi address ...
@ Found '192.168.1.2
192.168.1.14
192.168.1.20
192.168.1.21
192.168.1.109
192.168.1.137
192.168.1.240
192.168.1.254', updating /etc/hosts file ...

Leaving the script running in this condition I had, after 3 loops:
(/etc/hosts):

192.168.1.2
192.168.1.14
192.168.1.20
192.168.1.21
192.168.1.109
192.168.1.137
192.168.1.240
192.168.1.2
192.168.1.14
192.168.1.20
192.168.1.21
192.168.1.109
192.168.1.137
192.168.1.240
192.168.1.2
192.168.1.14
192.168.1.20
192.168.1.21
192.168.1.109
192.168.1.137
192.168.1.240
192.168.1.254 pi