I'm having an issue with GridFS while using MongoS for node-mongodb-native. I opened an issue in github, but it's really just a symptom of the problem. The error that doesn't get caught complains about ReadPreference not being set. I can't find a good example of setting ReadPreference with GridFS. Has anyone used the mongo driver in a sharded setup?
https://github.com/mongodb/node-mongodb-native/issues/686
They fixed this in the 1.1.5 driver update.