Discovering dynamic IP of firewall while away from home

I have a Cisco PIX 515 that is connected to my office cable modem.  The ISP has dynamic address allocation on the Outside interface of the PIX.  I have EasyVPN set up on the PIX for me to be able to VPN back into my network.  When I am on the road, I would like to be able to VPN back in the PIX to the office network.  However, the IP address can obviously change.  Getting a static address assignment from the ISP is currently not an option.

I am trying to find out if there is any way for me to find out what the PIX's Outside address while I am on the road.  There is a Windows 2003 network on the other side of the PIX with a domain controller and Exchange server if that helps.  In addition, somebody mentioned a utility called 'DNS to Go', but I have not found it.  I am also not sure, based on the utility's name, if it is what I need.

I can write a script that the home office staff can run to find out the address or have them run one of the 'What's My IP Address' websites to get the address.  However, I would like to be able to do this with no home office interaction.

Thank you for any suggestions.