MongoDB 1.6.0 is the fourth stable major release (even numbers are “stable” : 1.0, 1.2, 1.4, …) and is the culmination of the 1.5 development series.
The focus of the 1.6 release is scale-out. Sharding is now production-ready. The combination of sharding and replica sets allows one to build out horizontally scalable data storage clusters with no single points of failure.
A single instance of mongod can be upgraded to a distributed cluster with zero downtime when the need arises.