calling

var data = new[] {
new BsonDocument() { {_id:-1} }
};
BsonCollection col = ....
var safe = SafeMode.WaitForReplications(2, TimeSpan.FromSeconds(2));
col.InsertBatch(data, safe);

will throw:
MongoDB.Bson.BsonSerializationException : No idGenerator found for type: System.Int32

This used to work on version 0.5 with my own object CRUD test.