A one-liner which would get the current IP of that box you are NATed through:
wget -q -O - checkip.dyndns.org|sed -e 's/.*Current IP Address: //' -e
's/<.*$//' [1]
i'm not sure where that fits best, but IMO a shell-alias should suffice.
[1] some alternatives:
http://www.go2linux.org/what-is-my-public-ip-address-with-linux