Jan Wagner writes about using ethtool to poll the link state on network devices. While I'm pleased to see another happy ethtool user, it's a bit heavyweight for reading one bit of information and it's not ideal for scripting.
Much information about each network device is available under the /sys/class/net/name directory, including the carrier and speed attributes. (When the interface is down, some of these attributes cannot be read.) It's also possible to listen for link changes (rather than polling periodically) using the rtnetlink API, but this is probably not suitable for munin.