ipMonitor can monitor serial interfaces, but it's only going to tell you whether or not it responded to ping. You'd need to use the custom SNMP wizard to poll the appropriate MIB to get any details about line protocol.
If you're interested in more OOTB and advanced network-centric status and performance monitoring, you might want to consider checking out Orion NPM.
Orion I like, the price i can't afford...
Can any help me with finding the correct MIB for monitoring the serial interface status on a Cisco 2611 or 2621 router? I found a OID tree on Cisco's website, but with the hundreds listed, I don't know which is the one I need.
The generic IfOperStatus should work against your Cisco devices:
Simply use this within the "Custom SNMP (Wizard)" Monitor.