The 10gen team recently announced a "powerful new type of alert type available in MMS," that improves users' ability to manage MongoDB. Metric Min/Max Value alerts
provides alerting on a variety of host types and corresponding MongoDB performance metrics. For example, you can say “Alert me if any of my secondaries experience a repl lag of greater than <x> mins.”
You can sign up for MMS and read the rest of the post by heading over to the 10gen blog here.