cc: greg_10gen
Both of these settings use the setParameter framework and are only settable at startup. Available starting in 2.6.0-rc3.

• connPoolMaxShardedConnsPerHost - defaults to 200
  This is used for client read/write operations and represents the maximum number of connections held in the connection pool on mongos. The only reason to change this setting is if the total number of connections created is growing at the same rate as your client operations, this indicates connection churn and that this number should be raised for more efficient connection utilization.

• connPoolMaxConnsPerHost - defaults to 200
  This is used for cluster-internal connections between shards and the config servers and authentication. It would be unusual to need to adjust this setting unless your driver is not pooling connections and you're using authentication, which would require reauthenticating every connection.

Note, these settings do not prevent more connections from being created as needed, but the pools will not retain connections over this number.