Is there any way to use SNMP to poll an OID to find an interface's IP address?

I use a simple ADSL connection for a backup for our alarms.  So I open up our main m0n0wall (on
fiber) to allow pings from the ADSL m0n0wall, but it uses DHCP and every once in a while the ISP
screws with things and we end up getting assigned a new address (on a new subnet too).  Of course
the fiber m0n0wall won't permit pings from that address and it appears to be down.  I have to redo
the permit rule and routing info for the new subnet/address.  I'd like to monitor the IP address on
the WAN interface and if it changes I can trigger an alert.

Any ideas?  I done and SNMP walk and do see the address listed, but it appears in a table with the
row ID being the address itself.  Weird.

Thanks in advance.

Aaron