I have seen some other methods but this is my technique using the relatively lightweight sed:
export PS1=`/sbin/ifconfig eth0 | sed -n '0,/^s+inet addr:([0-9]+[.][0-9]+[.][0-9]+[.][0-9]+).*$/s//1/p'`':w$ '
Or you can use whatever interface you want instead of eth0. If left blank then it will display the first interface.