Assuming your router is running 12.3(14)T or higher, you could use the Embedded Event Manager to accomplish this. Using EEM, you could run the show ip route bgp command, store the results, then periodically recheck the table, and notify via syslog when there is a change. This would require an EEM Tcl policy. More on writing EEM policies in Tcl can be found at http://www.cisco.com/en/US/docs/ios/netmgmt/configuration/guide/nm_eem_policy_tcl.html , and a sample policy of mine which does something similar for DCHP leases can be found at http://forums.cisco.com/eforum/servlet/EEM?page=eem&fn=script&scriptId=1212 .
Admittedly, with a large table, this may not be doable with EEM right now. We are looking at introducing a new Route Event Detector in an upcoming EEM release that will make this a lot easier.
If you want to pursue the EEM route, and have questions, feel free to ask.
