Maybe someone knows a easy way and will tell us but there are 2 ways I suspect it can be done. I have not done this particular item.
First you could attempt to use the RMON command. This is a internal event monitor and can cause snmp traps and other actions based on changes in MIB variables. The challenge here is finding the correct MIB and then figuring out what name this command want you to call it. In most cases anything that can be displayed with a command you can find a mib for if you dig long enough with a MIB browser.
The new toy that I know can do it if you are brave enough is called Embedded Event Manager.
They have been adding a lot of features to this in the last few IOS releases. Since this is all based on scripts you can issue just about any command and take action based on the output. There are huge libraries of scripts for samples. This link is the best I have seen. This was a presentation from the cisco users group in dallas a couple of months ago.
http://dfw.cisco-users.org/zips/080302_Embedded_Event_Manager.zip
So far I only play with this is the lab until my TCL abilities improve.
