I was trying to specify the :include option to the to_json Document methods in order to include model associations in the generated json, but it turns out that it is not possible to do that on a collection, only when dealing with a single model instance.
Example:
MyModel.all.entries.to_json :include => :myrelationship

This ends up not including "myrelationship" in the generated json code at all. However, this works in this case:
MyModel.first.to_json :include => :myrelationship

Tested with mongoid 2.0.0 rc6