If you have access to another unix/OS X based machine on the net, you could setup an account there whereby your laptop would login there and create ssh tunnels for ARD. Then, it wouldn't matter if the laptop was behind a firewall. It might be rather slow depending on the network speeds of all three locations. One plus, is that your unix machine is the only IP you need to know to use ARD no matter where the laptop is located.

As for knowing the IP, just hit whatismyip.com or something similar from the laptop. You could even create a little C,python,perl app on the same unix machine that simply reports the laptops real IP when a connection is incoming and does nothing else.

I've used VNC this way. Well, without the automatic server login, though, but that can be done.