replica sets should support security and authentication.
sharding will use the same system later
sharding authentication support
mongod process stops when using --auth and the first connection fails
For replica sets: http://www.mongodb.org/display/DOCS/Replica+Set+Authentication
The replica set part of this is done for 1.7.5, bumping this case to 1.9.0 for the sharding part.
There is a separate sharding case.