On trying to restore against a sharded cluster, I ran mongorestore and got the following error message:

2015-02-24T12:50:47.149-0500 Failed: cannot do a full restore on a sharded system

The reason for this error - as I found by looking through the source - was that you need to specify the database when doing a restore against a sharded cluster. It would be helpful if the error message is clear about this.