Trying to execute a covered query using 'true' in the projection instead of '1' seems to make the query non-covered.

According to explain plans, the following query is covered:
db.coll.find({_id : "val"},{_id : 1})
But this one isn't:
db.coll.find({_id : "val"},{_id : true})

Same goes for the following queries where the collection has an index on the field 'a':
/covered/ db.coll.find(

,{_id : 0, a : 1})
/* non-covered */ db.coll.find(

,{_id : 0, a : true})
/* non-covered */ db.coll.find(

,{_id : false, a : 1})
/* non-covered */ db.coll.find(

,{_id : false, a : true})

Hinting the non-covered queries doesn't seem to help.